Provide *_result variants of reading functions with informative errors

[gasche]

Currently Ezjsonm does not provide users with the ability to properly
report errors. In particular, the error location inside the JSON
source is provided by Jsonm and thrown away by Ezjsonm.

In ocaml-mustache I had to remove the Ezjsonm usage and turn it into Jsonm directly to provide sensible error messages: rgrinberg/ocaml-mustache#63.

This PR provides *_result variants of reading functions
that (instead of failing in badly-documented ways) return an
informative error type in the case of reading failure, along
with simple functions to provide basic error messages to users.

Note: this PR uses the "result" type from the standard library, and as
a consequence it bumps the required OCaml version from 4.04 to
4.08. If this is judged too strong, we could use a compatibility
"result" module instead.

Finally: I would like to write tests for this PR, but I am not sure what would be the right approach. Any recommendation/preference?

[dinosaure]

We prefer an open polymorphic variant for errors (eg. [> error ]) to be able to compose it with others errors, can you update your PR? Otherwise, seems good. For tests, we use alcotest. About #63, I will try to find a solution to merge both worlds 👍.

[gasche]

Thanks for the feedback. I changed the PR to use open variants as suggested.

[gasche]

I just pushed some tests for from_string_result. I consider this PR complete on my end.

[dinosaure]

I'm currently on a merge of your PR and #42 👍.

[gasche]

Regarding #42, as mentioned I think that the proposed implementation is more complex than necessary. I am thinking of doing an implementation directly for Jsonm upstream, but I think that the version I suggested in the comments would basically be fine for Ezjsonm. I don't mind rebasing this PR if you merge some version of #42 before it.