Support drop for Python < 3.7: step 3 — remove unused types and save_*() functions

[leogama]

This is for the step 3 in #499:

• Clean-up the now unused type definitions if they are not in _reverse_typemap, to still allow unpickling of objects created with older combinations of python+dill versions.
• Remove unused save_<type>() functions, leaving the correspondent _create_<type>() constructors for backward compatibility.
• Clean-up _objects and tests.
• Unalias some types for internal usage, like BytesIO that is renamed to StringIO, or move the alias definition to _shims.