Compatibility with ocaml-h2

[314eter]

There are some differences with ocaml-h2 that still need to be addressed.

Error handling

Angstrom already has error handling, so I think there is no need to return a (header list, error) result if there is only one type of error.

Naming

Decoder.headers should be renamed to Decoder.decode_headers. I don't like the repetition in Decoder.decode_headers decoder very much, but it's probably clearer. Another possibility is to rename Encoder.encode_header to Encoder.header.

I prefer Encoder.change_table_size and Decoder.change_table_size_limit over set_capacity for both.

I think never_index is more descriptive than sensitive. It's perfectly possible to encode headers that are not sensitive with never_index. But it's not a strong preference, I'm okay with sensitive too.

Interface changes

The (optional) arguments of Decoder.create and Encoder.create are changed, but it's easy to adapt h2.

In #8 there is a bigger change to the interface, that makes Header.t a private type.

Header list order

The header list is not reversed anymore.

Table size changes

These are not implemented yet in h2.