Backend refactor (#120)

* remove spqlios, split cpu_ref and cpu_avx into different crates

* remove spqlios submodule

* update crate naming & add avx tests

poulpy-cpu-ref/README.md

@@ -0,0 +1,38 @@
+# 🐙 Poulpy-Backend
+
+**Poulpy-Backend** is a Rust crate that provides concrete implementations of **`poulpy-hal`**. This crate is used to instantiate projects implemented with **`poulpy-hal`**, **`poulpy-core`** and/or **`poulpy-schemes`**.
+
+## Backends
+
+### cpu-spqlios
+
+This module provides a CPU AVX2 accelerated backend through C bindings over [**spqlios-arithmetic**](https://github.com/tfhe/spqlios-arithmetic).
+
+- Currently supported: `FFT64` backend  
+- Planned: `NTT120` backend
+
+### Build Notes
+
+This backend is built and compiled automatically and has been tested on wsl/ubuntu.
+
+- `cmake` is invoked automatically by the build script (`build.rs`) when compiling the crate.  
+- No manual setup is required beyond having a standard Rust toolchain. 
+- Build options can be changed in `/build/cpu_spqlios.rs`
+- Automatic build of cpu-spqlios/spqlios-arithmetic can be disabled in `build.rs`.
+
+Spqlios-arithmetic is windows/mac compatible but building for those platforms is slightly different (see [spqlios-arithmetic/wiki/build](https://github.com/tfhe/spqlios-arithmetic/wiki/build)) and has not been tested in Poulpy.
+
+### Example
+
+```rust
+use poulpy_backend::cpu_spqlios::FFT64;
+use poulpy_hal::{api::ModuleNew, layouts::Module};
+
+let log_n: usize = 10;
+let module = Module<FFT64> = Module<FFT64>::new(1<<log_n);
+```
+
+## Contributors
+
+To add a backend, implement the open extension traits from **`poulpy-hal/oep`** for a struct that implements the `Backend` trait.  
+This will automatically make your backend compatible with the API of **`poulpy-hal`**, **`poulpy-core`** and **`poulpy-schemes`**.