reflect: derive correct Value method in panic messages
methodName was brittle in that it assumed exactly where
in the call stack the exported Value method is.
This broke since recent inlining optimizations changed
exactly which frame the exported method was located.
Instead, iterate through a sufficient number of stack entries
and dynamically determined the exported Value method name.
This is more maintainable, but slightly slower.
The slowdown is acceptable since panics are not the common case.