refactor!: simplify functor implementation

[lessp]

There's a bit going on in this PR.

One of the motivations behind these changes is that in order to create an implementation for BuckleScript, as there's no equivalent of dune's (modules_without_implementation), that had to be removed.

• Simplifies the Fetchify-functor a bit (very much inspired by @aantron's efforts from Luv).

• Adds some documentation (ODOC): https://github.com/lessp/fetch/pull/27/files#diff-e7aa0a2cc3a43c46e03abce08219a5d7R69

• Fixes an oversight where headers were not included in the request: 746fe7f#diff-979fa451f04b87e867ffba4b8812290dR68

• Included in this change, is also a breaking change which I'm wondering if it might be a good idea to break out into a separate issue. It's the fact that Body.toString returns Promise.t(string) instead of a string which was previously used for a simpler API, but might not be the best way forward: https://github.com/lessp/fetch/pull/27/files#diff-e7aa0a2cc3a43c46e03abce08219a5d7R7 Create a separate issue for this.

• Moves quite a lot of files around, especially esy-manifests have been separated. An example is that tests for fetch-core are now run from the fetch-core-manifest instead of a global esy-manifest.