Implement READ-LINE and WRITE-LINE

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.