Preliminary support for a :let command in the repl
Changed :let to work with an arbitrary number of `PDecl`s (though only one name at a time for now).

	It is no longer possible to leave off the type declaration. This will be fixed soon.
Handling more forms of definition.

	 In particular, it should now be possible to define multiple names at once, and to
	 leave off type declarations when the type can be inferred.
Removed some commented-out code
Added support for data definitions
Merge pull request #1361 from trillioneyes/repl-definitions

Repl definitions