refactor(developer): move file write out of WriteCompiledKeyboard

Stage one of the kmcmplib filesystem refactoring, as we move to writing
to buffers and allowing caller to write to disk. Moves responsibility
for writing data out of WriteCompiledKeyboard and into
kmcmp_CompileKeyboardFile. This means that there are now no file writes
in Compiler.cpp, except for the temp UTF16fromUTF8 (which I will also
change to a in-memory buffer "real soon now").

Note that responsibility for writing in kmcmp_CompileKeyboardFile is
very much temporary as we'll move it out of the library altogether in
a subsequent commit.