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

### 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`.