Stdlib: add popcount, nlz, nls, ntz and related bit count functions

[xavierleroy]

This PR adds classic bit counting functions to the Int, Int32, Int64 and Nativeint standard library modules. (Only Int64 is implemented at this point; I'll do the other modules when we agree on the API, implementation, and documentation.)

The new functions are the classic

• popcount: population count / Hamming weight
• nlz: number of leading zeros
• ntz: number of trailing zeros
  as well as three derived functions that I find interesting as well:
• nls: number of leading sign bits
• unsigned_bitsize: number of bits required for an unsigned binary representation
• signed_bitsize: number of bits required for a signed, two's complement binary representation.

The implementation is pure OCaml, using Hacker's Delight algorithms. I know we can get better performance using C with built-in functions, or inline asm, but speed is not the main goal of this PR. The emphasis here is on providing the basic functionality in the stdlib and on fixing the API.

[nojb]

Prior art: #11312 #2179.

[xavierleroy]

Both prior art shoot for performance on x86, emitting processor instructions and what not. I think we should first agree on the functionality (is it something we want to have in the stdlib?) and the API, before considering compiler support.

[c-cube]

Oh thank goodness, yes, please. Getting it in before making it fast is great, imho. This is going to be very useful for HAMTs and other persistent structures!

[redianthus]

I think this is a great addition and I agree that it is better to get this in first and worry about performances later.

In Wasm, they are called popcnt, ctz and clz (there is no equivalent of nls AFAIR).

In ocaml_intrinsics they are count_set_bits, count_leading_zeros and count_trailing_zeros.

I think using "count" rather than "number" is more standard here. But using ctz, count_trailing_zeros or any variation is fine IMO.