Typechecking logic for SIds is overcomplicated

[cd1m0]

The typing logic for SId is split between several functions: tcId, tcIdVariable, tcLetAnnotation, tcUserConstant, lookupFun, lookupVarDef, lookupTypeDef, tcIdBuiltinSymbol, tcIdImportUnitRef. Whats worse, there are 2 redundant styles of checking types:

a) Using an explicit stack of scopes (see lookupVarDef for an example)
b) A messy bunch of ifs all over the place. (see tcId for an example of this)

This logic is ugly, over-complicated and probably has a lot of buggy edge-cases with shadowing.

I think it would be cleaner if we had a single way to type-check SIds, just using a single explicit stack of scopes. In that case we would:

1. Unify lookupVarDef, lookupFun, lookupTypeDef and tcLetAnnotationId into a single lookupDef function.
2. Replace all the tc* functions with a single tcId function whose body will look almost exactly like the body of tcIdVariable
3. We will add a new BuiltinsScope type for all the builtins (its not clear yet exactly where that will go).
4. For every 2 types of DefSites we will need a test to determine how they get shadowed. For example for builtins and variables, we will need to see if a local variable called msg shadows the msg builtin. Similarly for variables and functions, we will need a test checking their precedence. And so on and so forth.

In practice, its probably better to first start with the tests in (4) and then begin implementing the code changes.