this adds the aPlonK cryptographic method alongside Semi-AVID and KZG+.

## changelog
- new feature `aplonk`:
  - the `algebra` module is compiled when either `kzg` or `aplonk` features are enabled
  - `algebra::scalar_product_polynomial` compiles only with `kzg`
  - the other `algebra::*` functions compile with `aplonk`
- `u32_to_u8_vec` has been moved to new `conversions` module which compiles when either `kzg` or `aplonk` features are enabled
- new `aplonk` module which compiles only when the `aplonk` feature is enabled
  - public structures
    - `Block`
    - `Commitment`
    - `SetupParams`
    - `VerifierKey`
  - public functions
    - `setup`
    - `commit`
    - `prove`
    - `verify`
  - internals
    - `ipa::Params`
    - `ipa::Proof`
    - `ipa::prove`
    - `ipa::verify`
    - `polynomial::compute_g`
    - `transcript::initialize`
    - `transcript::reset`
    - `transcript::hash`