unconvert: don't panic on source that uses cgo
unconvert: don't panic on source that uses cgo

Package loader parses the cgo-processed source,
so offsets and column numbers don't match the original
source.  This commit changes the internal code to track
the exploded token.Position values instead of raw integer
offsets, so that unconvert can at least report line numbers
of unnecessary conversions in cgo-using source files.

Updates #3.
mdempsky/unconvert