io.m:
Fix the mode declaration for io__gc_call now that higher-order
modes are implemented properly.
mercury_builtin.m:
Add declarations for call/4.
Remove mode declarations for call/3 with outputs before inputs,
since that isn't supported.
io.nu.nl:
Use library__version to get version number.
library.m:
Bump version number to 0.4.
list.m:
Add list__sort.
Comment out implied mode of list__length (I think that dates
from the time we didn't implement implied modes properly).