Add best::table, a hash table implementation

[mcy]

This change adds best::table, a Swisstable implementation providing both a map-like and set-like interface.

Also included are:

• best::hash, a generic hashing implementation, comparable to Rust's Hash trait.
• best::fxhash, an fxhash implementation using the above.
• best/base/arch.h, containing feature-test macros.
• best::option::expect, same as Rust.
• best::must and best::debug_must, for assertions.
• best::layout factories that take initializer lists, for dynamic layouts.
• best::mul for full-width multiplication.
• best::abridge symbol improvements.
• best::round_up_to_pow2, for rounding to a power of 2.
• best::ptr::skip, best::ptr::misalignment.
• best::span::as_bytes().
• best::is_pair for working with pair-like types.
• best::prefetch_for_read and best::prefetch_for_write, prefetch intrinsics.