Optimize .config writing.
Optimize .config writing.

The old note no longer seems to apply. Going through the defconfig part
of the test suite without verification in PyPy drops from ~5:50 to
~5:30, and line_prof shows some improvement for CPython too.

Passing the 'write' method around instead was a bit slower.

ulfalizer/Kconfiglib,ulfalizer/Kconfiglib