@aqjune @nunoplopes: See new tests at tests/lean/run/random.lean
We have two actions in `io`. By default, `io` uses the C++
random number generator, but we can force it to use a `std_gen` with
the action `set_rand_gen`.
def rand (lo : nat := std_range.1) (hi : nat := std_range.2) : io nat
def set_rand_gen : std_gen → io unit
@aqjune @nunoplopes: This commit adds basic support for random numbers.
It defines a random number generator interface, and an basic
implementation based on the Haskell one.
We can add more implementations in the future if neeeded.
The new test program has a few examples.
BTW, this is a pure Lean implementation.
If we need more performance we can provide an implementation
using C++.
