Implement `best::vec` and a host of things it requires

This PR adds:

best::vec, requires for best::str_buf and eventualy best::fmt.
best::layout, best::allocator, and best::malloc, to support memory management.
best::span and best::option get all of the nice Rust convenience helpers I felt like adding.
best::overflow and various other integer utilities.
best::unsafe now works more like Rust's unsafe.
New doc comment style, applied globally (except for best/meta which is currently a huge mess).
And lots and lots of tests.

mcy / best