[Moore] Add builtin for $urandom

[Scheremo]

First time I try to add a new Op, so let me know if I went wrong somewhere!

This PR adds a new builtin in moore for the $urandom system call.
Per the newest version of the spec, $urandom may be called with or with a integer seed argument, and always returns an integer produced by a pseudo-random number generator.

As such I added an importer conversion and tests for both cases; the allowed expressions are

moore.builtin.urandom
moore.builtin.urandom %arg : !moore.i32

I struggled a bit with constraining the output type of moore.builtin.urandom, since it is constant. I added

def MooreI32 :
  Type<CPred<
    "(::llvm::isa<::circt::moore::IntType>($_self) && "
    "::llvm::cast<::circt::moore::IntType>($_self).getWidth() == 32 && "
    "::llvm::cast<::circt::moore::IntType>($_self).getDomain() == "
    "::circt::moore::Domain::TwoValued)"
  >, "!moore.i32"> {
  let builderCall =
    "::circt::moore::IntType::getInt($_builder.getContext(), 32)";
}

in place to constrain an IntType to be exactly a moore.i32 for this purpose, but please let me know if there is a smarter way; this seems a bit cumbersome.

[fabianschuiki]

LGTM! Just a minor nit about moving the type constraint into MooreTypes.td. Thanks for adding this! 🙂

[Scheremo]

Move the new Type to MooreTypes.td and renamed to TwoValuedI32 and used it to construct the seed optional argument. Thanks for the quick and very helpful feedback!
If there is no other comments, I am okay with merging whenever you get to it.