Started adding sequences to the source

Change the Let AST to be

Let (maybe varN) exp exp

which should be minimally invasive. So (Let NONE e1 e2) is just (e1;e2). This
had the nice side effect of removing an instance of creating (Let "_" e1 e2)
from the parser.
Fix type soundness
Fix inference soundness
Translator fixes
Fix compiler
Merge branch 'sequences' into cenv_close

Conflicts:
	compiler/intLang3.lem
	compiler/intLang3Script.sml