Make Digest trait generic over output size

[Farukest]

Problem

The current Digest trait has a hardcoded 32-byte output constraint:

fn as_bytes(&self) -> [u8; 32];

This makes it impossible to properly implement hashers with different output sizes. For example, SHA-512 produces a 64 byte digest, but the trait forces truncation to 32 bytes.

Using truncated SHA-512 (first 32 bytes of SHA-512 output) is not the same as SHA-512/256, which uses different initialization vectors as specified in FIPS 180-4. This creates a confusing situation where implementers must choose between:

1. Implementing Digest with incorrect/truncated output
2. Not implementing Digest at all for 64-byte hashers

Proposed Solution

Add a const generic parameter to the Digest trait:

pub trait Digest<const N: usize = 32>: Debug + Default + Copy + Clone + Eq + PartialEq + Send + Sync + Serializable + Deserializable {
    fn as_bytes(&self) -> [u8; N];
}

This would allow:

• Digest<32> as default for existing 32 byte hashers (backward compatible)
• Digest<64> for SHA-512 and other 64 byte hashers
• Future flexibility for other digest sizes

Context

This issue was raised while implementing SHA-512 support in 0xMiden/crypto#692.

cc @huitseeker

[Nashtare]

As far as I remember this was also the issue behind abandoning the integration of Tip5 into winterfell a while back.