Add PRNG qsystem lib

[ss2165]

Module name: guppylang.std.random

Functions:
Note we should not use names from the python standard library random because they have different signatures and behaviours

@guppy.struct
class RNGContext:
  # can only be initialised once per program. Second init fails at rutime.
  def __init__(seed: int) -> "RNGContext": ...
  # init safely, returning None if already acquired.
  def init_safe(seed: int) -> Option["RNGContext"]: ...
  def random_nat(self) -> nat: ...
  def random_int(self) -> int: ...
  def random_float(self) -> float: ...
  def random_int_bounded(self, bound: int) -> int: ...

[qartik]

Module name: guppylang.std.random

Should random be namespaced under qsystem module or std?

[ss2165]

probably safer to go with qystem for now

[qartik]

def random_nat(self) -> nat: ...

is this just an alias for random_int with some type conversions?

[ss2165]

is this just an alias for random_int with some type conversions?

yup