std.Random: add functions with explicit index type

[InKryption]

Should address #13247; this doesn't make enumValue or shuffle consistent across architectures, but it does allow one to use the alternative function to specify an integer with a bit width independent of the architecture.

Also added some simple comptime checks to enumValue*, to take advantage of some possible invariants.

[InKryption]

I am going to push a commit to rectify doc comments, but for the record, all tests have passed:

[wooster0]

The other two functions shuffle and shuffleWithIndex seem to be a very similar case so maybe we can have the same docs for them too just like above? Linking between them and explaining why shuffle is not independent of the target.