Avoid/eliminate pickle for storing/loading a code table

[soxofaan]

PrefixCodec.save() (and .load()) currently use pickle to persist the code table to a file.
While this has the advantage that it handles Python data structures out of the box, it has a couple of disadvantages:

• it's python specific
• there are security related problems with pickle as noted by @KOLANICH in encode/decode data directly to/from binary file #2 (comment)

The most obvious alternative is JSON.

[KOLANICH]

You have options:

1. Try to utilize some well-known format having a Huffman tree within it. The benefit is that reusing Huffman trees from all files of that format is achieved automatically.
2. implement an own binary format. To prototype it I recommend Kaitai Struct DSL (it also supports sub-byte stuff, it is not necessary to be limited to whole bytes). It will generate only a parser though. It will still make sense to rewrite the result in vanilla python and mmaps (BTW, feel free to reuse https://github.com/prebuilder/fsutilz.py/blob/master/fsutilz/__init__.py#L8-L47 ).
3. Use some well-known binary serialization format, as CBOR. It would be less efficient, but more compatible. Also CBOR is not in the stdlib.
4. Use a text format, like JSON, but it would be inefficient