Rename `FixedWidthInteger._random()` to `FixedWidthInteger.random(in: UnboundedRange)`

[benrimmington]

@t-ae

If we want signed/unsigned integers to have common interface/implementation, simply renaming _random to random is an option.

This was rejected in the SE-0202 proposal.

Add static .random() to numerical types

There's also an issue with how UnboundedRange is currently implemented.

Gold-plating UnboundedRange

(This may be a more appropriate topic for Evolution, but here we go…) The text of SE-0172 never described a facility for unbounded ranges, but nevertheless the implementation ended up growing one. At the moment, it’s definitely creaky; it only works with the one subscript on Collection, not on anything that takes <T: RangeExpression>. This is not only annoying when composing the use of stdlib APIs, but also painful for writing your own APIs that use RangeExpression; this is especially true if i…