Remove explicit casts from some integer constants, to improve readability

Kremlin is careful to produce constant values with predictable behavior, despite ambiguities in C about integer promotion due to sizeof(int) and signed-ness of char.

Examples:

- (uint32_t)1U

- (uint32_t)0U

- (int32_t)1

- (int32_t)0

I think Kremlin could omit the casts in a number of cases, while still being certain that the ambiguous cases are well handled. I believe it is safe for constants < 16 bits in length, where it already emits the 'U' suffix for unsigned, or omits it for signed.

For larger constants, close to sizeof(int) on a 16-bit machine, the explicit casts will still be important.

Start by tracing KaRaMeL's constant-emission logic for the listed signed and unsigned integer forms. Compare generated C for constants below 16 bits with larger constants near the stated 16-bit-machine boundary; done means safe small constants no longer use explicit casts while larger ambiguous constants retain them.

Written by the indexing model from the issue text.