This PR adds typed Header API to cohttp-eio.

This is the first PR in this series. I will open additional PRs that will build on this PR and which will use the header api defined here. Future PRs will address changes required in Request, Response, (Server?) and Client modules. This is mostly to ease review and merge.

Usages of the new API can be seen in the header.md tests.

A Brief Overview

The header functionality implemented in the module Header aims to address the following core requirements,

1. Type-safe API to manipulate (add,update,find, remove, etc ) HTTP headers.
2. Extendable by end-users so that custom headers can be used while still following (1) above.

For (1) and (2) we define an extensible GADT -

type 'a header = ..

where 'a defines the header type. For example to define Content-Type header we define it as

type 'a header += Content_length : int header

This ensures that valid Content-Type header is an int.

The current PR provides implementation for three common HTTP headers, Content-Type and Transfer-Encoding and a type-unsafe, generic/catch all header aptly named H name. The plan is to increase this number in future PRs after this PR is merged.

Some headers are only applicable to either requests only or response only, as such they will be defined in Request and Response module respectively. Common headers to both Request and Response will be defined in the Header module itself.

Type 'a header = .. is also an extensible type so that end-users can define custom headers should they so desire, for example we can define Header1 and Header2 as such:

type 'a Header.header += 
  | Header1 : string Header.header
  | Header2 : float Header.header 

Tests in header.md contains a test for custom headers and its usage.

Lastly, the header values are not decoded during insertion time, they are encoded as a 'a Lazy.t values and are only decoded on demand. This ensures that we don't incur performance penalty for those headers which we may never use. The backing store of headers is a list so insertion is 0(1).

/cc @avsm @mseri