Allow utf8 in mappings

Allow utf8 in mappings

We were previously processing entries in mapping files (when
`--mappings-are-raw` is not given) with
`.decode('unicode_escape').encode('utf8')` to replace backslash escape
sequences in bytestrings with the utf-8 encoded characters they
represent. However, it turns out that `.decode
('unicode_escape')` assumes latin-1 encoding if it encounters non-ascii
bytes: https://bugs.python.org/issue21331. So this gave incorrect
results if non-ascii utf8 data was present in the mapping.

To fix this, we now add an extra layer of `.decode('utf8').encode
('unicode-escape')` in order to convert any non-ascii characters into
their backslash escape sequences. Then the subsequent
`.decode('unicode_escape')` only encounters ascii characters and gives
correct results.