Split mml grammar theory in two, isolating parse tree conversion functions.
Parse tree translation for the DtypeCons non-terminal.
Parse tree translation for the DtypeDecl non-terminal.
Introduce a ptree_linfix constant to simplify some ptree translation fns.
Parse tree translation for datatype declarations.
Cleanup utility functions in parsing directory.

In essence, this has moved all useful stuff to the mmlGrammar theory,
which is the base of the “hierarchy”.