More WIP on refactoring, focusing on decoupling foreign function definition from backend

git-svn-id: https://svn.mcs.anl.gov/repos/exm/sfw/stc/trunk@12737 dc4e9af1-7f46-4ead-bba6-71afc04862de
Disable pruning temporarily

git-svn-id: https://svn.mcs.anl.gov/repos/exm/sfw/stc/trunk@12738 dc4e9af1-7f46-4ead-bba6-71afc04862de
Temporarily disable pruning of builtins

git-svn-id: https://svn.mcs.anl.gov/repos/exm/sfw/stc/trunk@12740 dc4e9af1-7f46-4ead-bba6-71afc04862de
Add back in function pruning.

git-svn-id: https://svn.mcs.anl.gov/repos/exm/sfw/stc/trunk@12741 dc4e9af1-7f46-4ead-bba6-71afc04862de
More documentation and minor refactoring for CompilerBackend interface

git-svn-id: https://svn.mcs.anl.gov/repos/exm/sfw/stc/trunk@12742 dc4e9af1-7f46-4ead-bba6-71afc04862de