Prove that the CakeML grammar is unambiguous.

This fact has already been used in the repl-parser's correctness
statement, but I figured we could have it explicitly too.
Rename mmlG ("mini ML grammar") to cmlG everywhere.
Move files with "mml" in their names to versions with "cml"
Substitute "cml" for "mml" everywhere.

Hopefully this purges all mentions of miniML, preferring instead
CakeML.
Minor improvement to the parsing README.