Fix felt! macro to use Felt::from_u64_unchecked (instead of from_u32)

[greenhat]

What

As discovered in #357, we parse u64 constants as two i32 parts on the stack, which makes translating from_u64_unchecked calls to bitcast op non-optimal. So, temporarily, in #357 I switched for felt! macro to use Felt::from_u32.

How

The easiest way is to peek into the next translated op in the frontend, and if it's a Felt::from_u64_unchecked call translate u64 constant as one u64 value on the stack. Alternative would be to allow to alter n previous translated ops, i.e. convert push.i32.i32 to push.u64.

[bitwalker]

@greenhat The reason we were using u64 was so that we could support the full field element range, u32 only supports a small portion of it (maybe fine for our current use cases), but probably will need to switch back to u64.

I think with the new IR, we'd translate calls to Felt::from_u64_unchecked to the hir.constant operation, with a u64 operand, followed by a hir.cast operation to felt type - this would get constant folded to a hir.constant operation with a felt operand, and lowered to MASM as a field element literal, which I believe is what you want. The only thing we need to add is the Folder implementation for hir.cast.