Better support for OCaml syntax

This PR implements most of the ideas in #67.

Moves calls to createElement inside the component implementation, to allow consumers to not use JSX attributes, just using plain functions, e.g. let my_element = Greeting.make ~name:"John" ().
The change above carries some constraints:
- the ppx now keeps original Reason JSX ppx behavior, and wraps all children in a list
- children types need to become of type element list to avoid warnings for keys missing in cases where it's not needed.
- for components that pass children over, we need to re-enable spread operator ...
Removes the functionality that used to be needed for module signatures, as right now they can just be typed as regular functions with labelled args.
Simplifies the handling of component implementation arguments in the ppx. In most of internal ppx code we don't care about Nolabel args (as only final () is allowed), so we create a new type with only Labelled and Optional values to remove handling edge cases that never happen.

Todo

Maybe some (or all 😅 ) of these things can go in future PRs:

[x] Add support for let%component syntax
[x] Move all generated functions inside either an anonymous module (open struct ... end) or inside a top level binding let make = let .... to avoid them appearing externally
[x] remove interface test in ppx, doesn't make sense now and adapt Interface text (to explain no action is needed)
[ ] Add snake case version of all methods
[ ] Call setDisplayName when building with profile=dev Dune mode to keep module names in React browser extension
[ ] Better locations
[ ] Make it so that let make ~foo () bar = fails to build