Removed trailing whitespace from all .sml and .sig files.

This affects over 900 files and was done using emacs's delete-trailing-whitespace
function in batch mode. Building the system with Poly/ML and Moscow ML seems to
work, so I'm hoping these changes don't break anything. Please complain if
they do!

An implementation of a type of sets of strings optimised to provide a
prefix searching function that will be useful in my upcoming
modification of the way the lexer works.