This PR optimizes type instantiation as follows:

• During instantiation of a symbol, if the type of the symbol has already been resolved and if that type is one that is never affected by instantiation, just return the symbol itself.
• During instantiation of a type reference, union type, or intersection type, only create a new type if something changes when instantiating the constituent type list.

Haven't measured effects on performance, but it certainly removes unnecessary work and causes fewer symbols to be created.