Add a crate which wraps `getrandom` but always compiles

Add a crate which wraps `getrandom` but always compiles

In the next commit we'll drop the `ahash` dependency in favor of
directly calling `getrandom` to seed our hash tables. However,
we'd like to depend on `getrandom` only on certain platforms *and*
only when certain features (no-std) are set.

This introduces an indirection crate to do so, allowing us to
depend on it only when `no-std` is set but only depending on
`getrandom` on platforms which it supports.