rename TypeVar and friends

[gelisam]

The word "TypeVar" is used to mean two different things in our codebase:

1. a type unification variable. The type is MetaPtr (itself a confusing name because it's not one of those mortise/tenon Ptrs, and because this name is inconsistent with KindVar, the kind-level equivalent), but the TypeStore maps each MetaPtr to a TVar (itself a confusing name because it means Type Variable, not Control.Concurrent.STM.TVar), whose fields are set via functions named setTVLinkage and setTVLevel
2. a scheme variable. The type is just Natural (though I have just opened a PR to wrap it in a SchemeVar newtype), but ETypeVar (introduced by prepareTypeVar) is the primitive macro for expanding a symbol to a scheme variable.

I would like to clean up this confusing terminology. How about the following renamings?

1. MetaPtr -> TypeVar
2. TVar -> TypeVarInfo
3. ETypeVar -> ESchemeVar
4. prepareTypeVar -> prepareSchemeVar

[gelisam]

Or maybe this?

1. MetaPtr -> TypeVar
2. SplitCorePtr -> SplitCoreMortise etc.

[gelisam]

while we're at it, ExprF should probably be called StxF, since syntax objects are used to construct patterns etc., not just expressions.