Users are unable to use the callbacks inside `internal/ice`.
Even though we alias things like OnSelectedCandidatePairChange
are unusable since in the package they use `ice.Candidate` instead of
`ICECandidate`
Moved ice related files to internal/ice. Added ice.go and
ice_go.go files containing aliases to internal/ice. Added
internal/tools/gen/genaliasdocs.go helper script for generating the
documentation for these aliases. Dropped ice
prefix from file names in internal/ice package.
