Moved Zlib to where it's used.
Added stubbed Encode module.
Added another custom exception.
Added InvalidTupleArity.
Added encoding tests for atoms, bools, nothing.

Also added tests for utility funcs.
Removed unused type.
Added const and new functions.
Added encoding tests.
Started work on small tuple decode.

Decided to wait until encoding for tuples was done so I could write more tests;
the Python source that I'm using as the basis for the Julia conversion doesn't
have enough tests for tuples.
First pass at encoding implementations.