add node id to instrumentation on check_expr_with_unifier
Fix `type_is_sized` (issue 10126).

Driveby: put in `add_builtin_bounds` method as place to put debug hooks.
Code cleanup.  Not necessary to land.
Code cleanup.  Not necessary to land.
Revise the typeck kind analysis to carry along more context info.

Tracks both a broad context (is cursor within scope of a
type-expression) and a narrow context (is cursor within immediate
context of a match discriminant).

Almost certainly should be revisited in the future.
Various changes to work towards getting DST bootstrapping.