Leverage type information available in type switch.

[PurelyApplied]

Within a type switch, non-default blocks have definite type information that remains obfuscated by interface. That information could be carried within that block and those dominated by that case entry for better reporting. This could possibly also be applied to type assertions, though nontrivial effort would be required to support typ, ok := foo.(bar) style type assertions.

A sample of (suppressed) test cases will be introduced by #149.