Implement non-deterministc mode

Implement non-deterministc mode

Part of #653

This allows us to properly implement getrandom(),
which unlocks the default HashMap type (e.g. HashMap<K, V>)
with RandomState)

This commit adds a new '-Zmiri-seed=<seed>' option. When present,
this option takes a 64-bit hex value, which is used as the seed
to an internal PRNG. This PRNG is used to implement the 'getrandom()'
syscall.

When '-Zmiri-seed' is not passed, 'getrandom()' will be disabled.