Fully support generics for the C target

[petervdonovan]

• handle type arguments that are more syntactically complex than just a single identifier
• propagate values for type arguments across multiple levels of instantiation hierarchy
• use #defines instead of string substitution so that behavior is consistent with that of C preprocessor and so that types in LF files can be defined in terms of generics? (This was discussed multiple times and I vacillated but I think this will be the better choice)
• add more integration tests for generics