feat: felt creation from bytes is infallible

[Oppen]

Expose 4 ways to create a field element from arbitrary bytes:

• from_bytes_be/from_bytes_le: take exactly 32 bytes and interpret them as a big/little endian number of 256 bits and then build a field element from it, modulo STARK prime (in whatever representation Stark252PrimeField uses, currently Montgomery);
• from_bytes_be_slice/from_bytes_le_slice: take any number of bytes and interpret them as a big/little endian number modulo STARK prime by decomposing into 256 bits integers and multiplying by the appropriate factors.

Pull Request type

• Feature

What is the current behavior?

Current behavior mimics that of lambdaworks, where depending on the length of the slice passed to from_bytes_xe (where x is either b or l) one of the following happens:

1. Length is exactly 32 bytes: conversion succeeds and the bytes are interpreted as a single 256 bits integer that is then stored modulo P;
2. Length is less than 32 bytes: conversion fails and an error is returned;
3. Length is more than 32 bytes: excess bytes are ignored, and 32 bytes are interpreted as a single 256 bits integer that is then stored modulo P.

What is the new behavior?

Now the functionality is divided in two:

• One family for exactly 32 bytes;
• One family for arbitrary length, where:
  1. If length is exactly 32 bytes, we see the old behavior;
  2. If length is less than 32 bytes, we 0-pad the most significant bytes to fit 32 bytes;
  3. If length is more than 32 bytes, we split the number in 32 bytes chunks (0-padding when needed) and compute the number as ∑Di*2^i where Di is the i-th "digit" of 512 bits units. This should be exactly equivalent to the following code (for little endian):

     let x = BigUint::from_bytes_le(bytes);
     let x = x % P;

     This way, we get the same result but without allocations on the heap. The powers of 2 are computed on demand by multiplying a constant.

Does this introduce a breaking change?

Yes. The signature of the from_bytes_xe functions change, they no longer return an error.