Initial union implementation

Still untested, but: make union declaration implicit (so no longer
inferred from 'enum' use), and initial implementation of type
inference.

Preparatory work for union literals

Contains horrible parsing hacks that transmute function calls and
enum literals to union literals if appropriate. Pending the
implementation of the tokenizer the AST can be constructed more
neatly.

Reimplement namespaced identifier to support polymorphic args

This is somewhat of a temporary hack, as a namespaced identifier should
not really refer to types or polymorphic arguments. But we need a
tokenizer and a prepass to properly distinguish identifiers from types.
So it works, but error messages may be cryptic.