Enable WASM-compat and monitor it in the CI (#142)

* fix: Use `target_pointer_size` conditional compilation

There are some parts of the code where is needed to de/serialze
`usize`s. These, have sizes that vary depending on the target
achitecture the code is compiled for.

Hence, this adapts the de/serialization to the specific pointer size for
which the crate is being compiled to.

* change: Support WASM-compatibility and polish Cargo.toml

In order to support Wasm-compat and to simplify and improve `Cargo.toml`
readability, the follwing changes have been made:

- All the deps that can use `parallel` feature, do so. As `rayon`
  supports non-threaded targets with a fallback option. See: https://docs.rs/rayon-core/1.12.1/rayon_core/index.html#global-fallback-when-threading-is-unsupported
- `ark-grumpking` has been brought to `0.5.0-alpha.0` as `0.4.0` appears
  to not be in `crates.io` anymore. See: https://crates.io/crates/ark-grumpkin/versions
- By default, the crate uses `"ark-circom/default"` which selects the
  `wasmer/sys` feature such that it knows where wasmer is
  suposed to be run`.
- Added a `wasm` feature which forces `ark-circom/wasm` to be used
  instead. Which internally selects the `wasmer/js` backend to be used
  such that in-browser execution is possible.
- Added `getrandom` with `js` feature as dependency when `wasm32-unknown-unknown` target is selected such
  that compilation of the crate for testing or simply building is possible. Notice that with `wasi` and other wasm targets,
  this is not the case as they're automatically supported.
  For more info, please check: https://docs.rs/getrandom/latest/getrandom/#webassembly-support

* feat: Support WASM-compatibility tests in CI

Add support for both testing the build of `sonobe/folding-schemes` for
WASM-targets and also, it's build as a dependency for a WASM-crate.

This includes a build job for the three main supported rust-WASM targets
and the same but for a thrid crate creted on-the-fly which uses
`sonobe/folding-schemes` as a dependency.

* chore: Add README docs about WASM-compat & feats

* ci: don't run WASM-compat job if PR is draft

* chore: depend on `arnaucube/circom-compat` fork.

Since https://github.com/arnaucube/circom-compat/pull/2 was merged, we
can already switch to it as we were depending before.

* chore: minimal build/test instructions

* fix: CI typos

* fix: Update CI to use correct feature sets

* fix: `ark-grumpkin` versioning issues

As mentioned in
https://github.com/privacy-scaling-explorations/sonobe/issues/146
there's a big issue that involves some dependencies of the crate.

As a temporary fix, this forces the workspace to rely on a
"non-existing" version of `ark-grumpkin` which is immediately patched at
workspace-level for a custom version that @arnaucube owns with some
cherry-picked commits.

While this allows the CI to pass and crate to build, a better solution
is needed.

* fix: Clippy CI avoiding --all-targets

* fix: use `wasm` feat only with folding-schemes

README.md

@@ -37,10 +37,41 @@ Available frontends to define the folded circuit:
 
 ## Usage
 
+### Build & test
+You can test the library for both, WASM-targets and regular ones.
+#### Regular targets
+Tier-S targets allow the user to simply run `cargo test` or `cargo build` without needing to worry about anything.
+**We strongly recommend to test using the `light-test` feature.** Which will omit the computationally intensive parts of the tests such as
+generating a SNARK of 4~5M constraints to then verify it.
+
+#### WASM targets
+In order to build the lib for WASM-targets, use the following command:
+`cargo build -p folding-schemes --no-default-features --target wasm32-unknown-unknown --features "wasm, parallel"`.
+Where the target can be any WASM one and the `parallel` feature is optional.
+
+**Trying to build for a WASM-target without the `wasm` feature or viceversa will end up in a compilation error.**
+
 ### Docs
 
 Detailed usage and design documentation can be found at [Sonobe docs](https://privacy-scaling-explorations.github.io/sonobe-docs/).
 
+### WASM-compatibility & features
+
+The `sonobe/folding-schemes` crate is the only workspace member that supports WASM-target compilation. But, to have it working, `getrandom/js` needs
+to be imported in the `Cargo.toml` of the crate that uses it as dependency.
+```toml
+[dependencies]
+folding-schemes = { version = "0.1.0", default-features = false, features = ["parallel", "wasm"] }
+getrandom = { version = "0.2", features = ["js"] }
+```
+See more details about `getrandom` here: https://docs.rs/getrandom/latest/getrandom/#webassembly-support.
+
+Also, notice that:
+- `wasm` feature **IS MANDATORY** if compilation to WASM targets is desired.
+- `parallel` feature enables some parallelization optimizations available in the crate.
+- `light-test` feature runs the computationally-intensive parts of the testing such as the full proof generation for the Eth-decider circuit
+of Nova which is approximately 4-5M constraints. **This feature only matters when it comes to running Sonobe's tests.**
+
 ### Folding Schemes introduction
 
 Folding schemes efficiently achieve incrementally verifiable computation (IVC), where the prover recursively proves the correct execution of the incremental computations.