Having the OS/runtime provide a cryptographic entropy source is the
right thing to do, and we need it to initialize the default random
number generator.
This adds `rand_bytes(dst: []byte)` which fills the destination buffer
with entropy from the cryptographic random number generator. This takes
the "simple is best" approach and just directly returns the OS CSPRNG
output instead of doing anything fancy (a la OpenBSD's arc4random).
Using a constant-time comparison is required when comparing things like
MACs, password digests, and etc to avoid exposing sensitive data via
trivial timing attacks.
These routines could also live under core:mem, but they are somewhat
specialized, and are likely only useful for cryptographic applications.
