Rework HashMap internals and add RNG support

The internals of HashMap, and in particular the hashing logic, were
broken. Internally the VM reused Rust's DefaultHasher type for hashing
values. This type is mutable. When storing a HashMap in a constant,
concurrent access to this HashMap could result in wrong hashes being
produced, as all threads use the same DefaultHasher.

To solve this, Inko takes a similar approach as Rust: we provide a
RandomState type, which can be used to create a DefaultHasher. A
DefaultHasher now takes two keys as arguments, used for seeding the
hasher. The RandomState type generates two keys randomly, similar to
Rust. The hash seeds are generated by taking a thread-local randomly
generated number, then incrementing it (wrapping around on overflow).
This ensures that it is very unlikely for two different HashMaps to use
the same seeds, making certain hash attacks [1] more difficult.

Random number generation is provided by the std::random module. This
module provides methods for randomly generating integers, floats, and
bytes. Integers and floats can also be generated in a given range, for
example:

    import std::random

    random.integer_between(min: 0, max: 10)

[1]: https://github.com/rust-lang/rust/issues/36481 and
     https://internals.rust-lang.org/t/help-harden-hashmap-in-libstd/4138/18