Typechecking logic for `SId`s is overcomplicated

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 `SId`s, 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.