properly parse ASN.1 in .SF [closes #47]

v1: s/signature(s)/signer(s)/

add FIXMEs for missing MD5 support for .MF/.SF digests

partially support MD5 (unsafe)

split off replace_apk_signing_block() from clean_apk()

add more docstrings [closes #57]

+ extract_meta() wrapper; return tuples

small fixes

add --signed-by [closes #56]

re-add some "type: ignore" comments still needed

remove "type: ignore" comments no longer needed w/ recent mypy

consider EC key size < 224 unsafe

CI: install coverage

CI: test coverage

make: coverage++ [skip ci]