This is implemented on top of READ-FILE and WRITE-FILE which avoids
new dependencies in the C code.
* fth/file.fth: New file.
* fth/loadp4th.fth: Load it.
* fth/t_file.fth: New tests. Some tests fail and some are commented
out due to missing functionality. There's an actual bug in SAVE-INPUT
which I know how to fix.
* build/unix/Makefile, build/mingw-crossbuild-linux/Makefile (test):
Run test in t_file.fth.