pseudo random number generator effect

[hehelego]

Implement a PRNG effect reifier and a programming language with PRNG effect.

TODO:

• PRNG language
• interpretation of PRNG language programs in guarded interaction trees
• soundness of the interpretation with respect to the operational semantics
• logical relations for safety
• interpretation and safety of the combination of PRNG+IO tape
• documentation for the new effect

Maybe out of scope:

• a language using PRNG + IO tape. (difficulties in state projection).
• take an arbitrary guarded interaction tree as the state update function on PRNG state creation.