store: fix many single-byte ops to use slicing in _auxencode

py3: add "b" prefix to string literals related to module policy String literals without explicit prefix in __init__.py and policy.py are treated as unicode object on Python3, because these modules are loaded before setup of our specific code transformation (the later module is imported at the beginning of __init__.py). BTW, "modulepolicy" in __init__.py is initialized by "policy.policy". This causes issues below; - checking "policy" value in other modules causes unintentional result For example, "b'py' not in (u'c', u'py')" returns True unintentionally on Python3. - writing "policy" out fails at conversion from unicode to bytes 62939e0148f1 fixed this issue for default code path, but "policy" can be overridden by HGMODULEPOLICY environment variable (it should be rare case for developer using Python3, though). This patch does: - add "b" prefix to all string literals, which are related to module policy, in modules above. - check existence of HGMODULEPOLICY, and overwrite "policy" only if it exists For simplicity, this patch omits checking "supports_bytes_environ", switching os.environ/os.environb, and so on (Yuya agreed this in personal talking)

py3: drop unused aliases to array.array which are replaced with bytearray