Closes #4.

It seems the size check would often fail on some fonts as the size of the compressed section provided by the header might not take into account the 4-byte alignment of the section. The comparison is now done between the actual consumed input size after processing the compressed section (the input buffer is consumed until after the potential padding bytes), and the size reported by the header aligned to the next multiple of 4.

The spec seems to indicate that the compressed section should always be padded to 4 byte alignment if followed by an extended metadata block or a private data block, but I'm not totally sure if it enforces this alignment when the compressed block is the last one. However on the font files I checked, the compressed block was always padded even if nothing followed.

This PR also add some derives on the public error type that might be useful to the crate consumers.

I tested with a few fonts (notably those listed in #4) and this seems to be working properly.