Add (user-defined) module features

[mtshiba]

Add (user-defined) module features. For more information, see the documentation.

TODOs:

• [ ] Let the compiler compiles modules in order according to the dependencies
• [x] Allow API references across modules
• [x] Aggregate method definitions distributed in each module
• optional
  • [ ] Allow parallel compilation

[mtshiba]

Implemented in v0.5.0.