Use runtime.CallersFrames

This release updates the implementation to use runtime.CallersFrames to examine stack frame data. CallersFrames was added in Go 1.7, so this package no longer supports versions of Go prior to 1.7. If you need to use Go 1.6 or earlier, you should pin this package to v1.6.0.

Go 1.9 changed the way stack data is returned from runtime.Callers. The good news is that we can now see stack information for inlined functions. The bad news is that inspecting stack data is generally slower and requires more memory than before.