[file.streams] Change sub-clauses to rSec2, similar to [string.streams]

[string.streams] and [file.streams] follow very similar structure (an introduction followed by streambuf, input stream, output stream and bidirectional stream class templates) but are nested entirely differently. Under [fstreams] we have an entirely flat hierarchy where the definition of `basic_streambuf` is at the same level as its own members.

This change makes [file.streams] consistent with [string.streams] by promoting [filebuf], [ifstream], [ofstream] and [fstream] to `rSec2`.

Also promote [c.files] to `rSec1`, I don't think `<cstdint>` should be nested underneath the sub-clause for `basic_fstreambuf` and `basic_fstream` etc.