start on top-level semantics for standalone programs

also, refactoring out things that aren't REPL-specific, and some
additions/changes to the grammar (the parser proofs will need to be
updated)
remove ptree_REPLPhrase_def from compute_semanticsLib
remove mention of REPLPhrase from gramPropsTheory
remove cmlParseREPLPhrase from cmlParseTheory
remove REPLPhrase_OK and fix TopLevelDecs_OK
update parser completeness
open the new semantic printTheory in compiler/proofs

(its contents were previously provided by replTheory)
Merge remote-tracking branch 'origin/standalone'

Conflicts:
	semantics/replScript.sml
I don't think this was a real conflict - just a whitespace error that
got corrected on master prematurely.