Remove availability restrictions from these internal functions

This is possible because this replaces
* `InlineArray` with `internal _InlineArray`
* `UInt128` with `internal _UInt128`

And the replacements are not availability-restricted.

We don't need to do the same thing with `MutableSpan`
because that back-deploys to Swift 5.0, which is as
far back as the stdlib builds anyway.

Author: tbkka