mod2c:
Change mod2c so that it outputs the appropriate macros required
for the use of assembler labels.
Note that mod2c is not model-dependent anymore (instead the
macros are model-dependent), so we're now only a small step away from
abolishing mod2c altogether.
mod2c, mgnuc.sh, ml.sh:
Add new grades fast.gc, reg.gc, etc. for conservative garbage
collection. Remove the --gc option from ml, since that information
is now part of the grade.
mgnuc.sh mod2c mod2init.sh:
Moved these scripts from the code directory to the scripts directory.
Changed mgnuc to use the MERCURY_C_INCL_DIR environment variable.
Changed mod2init to use MERCURY_MOD_LIB_DIR and MERCURY_MOD_LIB_MODS.
Changed ml to use MERCURY_C_LIB_DIR rather than MERCURY_CLIB_DIR.
