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

folding-schemes/src/folding/hypernova/mod.rs

@@ -593,9 +593,21 @@ where
             return Err(Error::MaxStep);
         }
 
-        let mut i_bytes: [u8; 8] = [0; 8];
-        i_bytes.copy_from_slice(&self.i.into_bigint().to_bytes_le()[..8]);
-        let i_usize: usize = usize::from_le_bytes(i_bytes);
+        let i_usize;
+
+        #[cfg(target_pointer_width = "64")]
+        {
+            let mut i_bytes: [u8; 8] = [0; 8];
+            i_bytes.copy_from_slice(&self.i.into_bigint().to_bytes_le()[..8]);
+            i_usize = usize::from_le_bytes(i_bytes);
+        }
+
+        #[cfg(target_pointer_width = "32")]
+        {
+            let mut i_bytes: [u8; 4] = [0; 4];
+            i_bytes.copy_from_slice(&self.i.into_bigint().to_bytes_le()[..4]);
+            i_usize = usize::from_le_bytes(i_bytes);
+        }
 
         let z_i1 = self
             .F