Replace frozenset with compat.UniqueFrozenset

This is not a trivial s/frozenset/compat.UniqueFrozenset/, but rather
only replaces “frozenset” where appropriate. Most of the places are
“static” information that doesn't change after the module has been
loaded.

Some docstrings and code formatting (e.g. empty lines) issues are
addressed as well. Some lines got too long and were wrapped.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>