lowering: Canonicalize to builtins for global assignment

[Keno]

This adjusts lowering to emit setglobal! for assignment to globals, thus making the = expr head used only for slots in CodeInfo and entirely absent in IRCode. The primary reason for this is just to reduce the number of special cases that compiler passes have to reason about. In IRCode, = was already essentially equivalent to setglobal!, so there's no good reason not to canonicalize.

Finally, the = syntax form for globals already gets recognized specially to insert convert calls to their declared binding type, so this doesn't impose any additional requirements on lowering to distinguish local from global assignments. In general, I'd also like to separate syntax and intermediate forms as much as possible where their semantics differ, which this accomplises by just using the builtin.

This change is mostly semantically invisible, except that spliced-in GlobalRefs now declare their binding because they are indistinguishable from ordinary assignments at the stage where I inserted the lowering. If we want to, we can preserve the difference, but it'd be a bit more annoying for not much benefit, because:

1. The spliced in version was only recently made to work anyway, and
2. The semantics of when exactly bindings are declared is still messy on master anyway and will get tweaked shortly in further binding partitions work.