Make encoding and decoding roundtrip correctly

THIS IS A BREAKING CHANGE

Leading nulls are now encoded as "0", followed by a count.
For example, `b"\x00"` becomes "01".
For example, `b"\x00" * 63` becomes "0z01".

Applications that rely on automatic stripping of "0z"
must now perform this stripping themselves
based on their knowledge of their problem domain.

Applications that rely on the `minlen` parameter
must now pad the output themselves
based on their knowledge of their problem domain,
and must remove references to the `minlen` parameter.

Applications that rely on leading null bytes being stripped
must now strip leading null bytes themselves.

Fixes #18

.gitignore

@@ -5,3 +5,6 @@
 build
 dist
 
+/.idea/
+/.venv/
+/.coverage*