Merge pull request #25 from velour/distribute_types

Simplify the AST's representation of types.
Change fields named "Name" to "Identifier."

Getting the name via s.Name.Name was too annoying. Now these fields
are embedded Identifier types, so s.Name can be used directly.
Add a simple symbol table type.

Currently, it's populated with dummy declarations for the universal
block's predeclared types, and with the package block's declarations.
Probably needs more thought.