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

[t-ae]

@benrimmington

The underscored API is non-public according to the standard library convention. The pull request discussion and commit message explain why it was added.

I see.
It is originally added as a part of FixedWidthInteger but the requirement was removed. Now it's used only in RandomNumberGenerator.next() despite its usefulness.

@benrimmington

As for overloading .random(in:) and .random(in:using:) to support UnboundedRange , there might still be confusion between 0 ... .max and .min ... .max for signed integers.

Signed integer confusion is conceivable. For them, PartialRangeFrom version may be well fit.

But at least we can have UnsignedInteger.random(in: UnboundedRange).
For full range generation we must choose UInt.random(in: 0 ... .max) or Random.default.next(). The former is I described in the first post and the latter will be renamed to longer name.

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

It seems Random API doesn't encourage using Random.default explicitly. So having alternative for Random.default.next() may fit the policy.