feat: support varlen hex strings in digest macro

[Mirko-von-Leipzig]

Somewhat mutilates/extends the digest! macro from #984 to add support for variable length hex strings.

Previously, the macro only supported hex strings of felt-length multiples. See this comment. Variable length is supported by manually unrolling the hex characters into u64/felts in reverse. This supports shorter lengths as the initial values will remain zero.

I also moved the Felt import inside the macro so that callers of digest! no longer have to import Felt.

I think this PR also fixes a LE/BE bug, where digest!("0x123")' maps to 0x1230000...0instead of0x123. Though I might just be misunderstanding the try_from` implementation it utilised. edit - indeed my misunderstanding - no bug

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Possible future work

Currently this macro will give a compile time error IF we assign it to a constant, but won't if its not assigned to a constant.

// Compile time error (missing 0x prefix)
const EXAMPLE: Digest = digest("ABC");

// Runtime error.
let example = digest!("ABC");

We can also force the latter to a compile time error by moving the const allocation inside the macro itself i.e. by adding

const DIGEST: $crate::Digest = ...;
DIGEST

to the end of the macro. This has further implications though, namely:

1. The macro can no longer accept non-const literals
2. I think the current macro insides need to move outwards into a pub const fn. Which isn't so bad because this then supports the cases from (1)

I can add this in if we want it.

[Fumuran]

Looks awesome!
Very clever approach, I'll keep it in mind for the future.

[bobbinth]

Looks good! Thank you! I left one nit inline.

We can also force the latter to a compile time error by moving the const allocation inside the macro itself i.e. by adding

I think it is fine as is for now. We can create an issue with this potential improvement (could be a "good first issue").