Remove warnings while compiling.

[dannywillems]

Because warnings are evil and are ugly.
Some comes from eliom_form about type inference, some comes from cmx missing for Ocsipersist. some from unused rec flag, etc.

[vasilisp]

Warnings are just warnings, and we have looked at the remaining ones.

We would need to do ugly things to get rid of the type inference warnings. I personally prefer the warnings as a reminder to get to #459 sooner.

The cmx should be easy to fix in Ocsigen Server.

The rec flag warning is the fault of ppx_deriving_json. If I remember correctly, it was not possible to annotate let bindings to prevent the warning.