native: annotate module-level values; discarded matches run in place

[simontreanor]

mypyc's front end rejected two shapes that a real multi-module program produces. This fixes both.

• Module-level values that another module reads failed with "Cannot determine type". Native mode now writes the type down:
  • _X: X = X() for a nullary singleton;
  • the inferred type on the first assignment of a top-level value that's bound once, when Python can spell it.
• A unit match in statement position was hoisted into a temp that only ever received unit calls, and mypyc rejected it with "has inferred type None". The arms now evaluate in place.

With both fixes, a real multi-module program gets through mypyc's type checking locally. It stops only at build_ext on a machine without a C compiler.

Tests: native_annotates_module_level_values_for_other_modules and native_evaluates_a_discarded_match_in_place. ROADMAP and INTERNALS are updated.