Implement sections inside modules.

[Drup]

There are two steps here:

• inside normal modules. This simply needs a refinement of the slicing pass.
• inside functors and first class modules. This is difficult.

If we ignore the typing difficulties related to functors (See https://github.com/Drup/eliomlang/issues/4), there is also a difficult for runtime execution. The number and content of sections must be static, in order to be synchronized between client and server. It could be possible to use fragments to implement those specific sections.

[Drup]

Done for "flat" modules. It will be forbidden inside functors for now.

[Drup]

Closing in favor of #17