Refactor and simplify

[colin-kiegel]

I merged the internal buffers enum [] and items [] into one buffer.

This simplifies the macro a bit and should prepare for additional complexity from #5 struct-like-variants.

Note: This PR is based on PR #8, I can rebase / change this one, if the previous one is not ok.

The changes are in the latest commit: 37dcff8

[tailhook]

Okay, I've merged in #10, you may now rebase this on top of the current master.

[colin-kiegel]

done.

Additionally I made the testcases a bit more stable to upstream changes.

[colin-kiegel]

I switched from std::io::Error to the following errors, which implement PartialEq

std::num::ParseFloatError;
std::str::Utf8Error;
std::string::FromUtf8Error;

[tailhook]

Looks good. Thanks!