Fixed ambigious reverse mappings. Added many new mappings. Import mapping
is no longer applied to modules already mapped with full name mapping.
Added tests for compatible pickling and unpickling and for consistency of
_compat_pickle mappings.
or dumping pickles with a 2.x-compatible protocol, in order to make data
sharing and migration easier. This behaviour can be disabled using the
new `fix_imports` optional argument.
