common: improve base64/32hex interface and error codes

src/common/base32hex.c

src/common/base32hex.h

src/common/base64.c

src/common/base64.h

src/common/errcode.c

src/common/errcode.h

src/libknot/binary.c

src/tests/common/base32hex_tests.c

src/tests/common/base64_tests.c