Module macros

Redefinition of the macro_exported macros to avoid namespace colision when the macros have the same name as modules.

An example of that is the crate::error module, and the error macro.

Macros

• bail_err
  TODO: Document
• call
  Helper macro to call a function with arguments
• define_unbox
  TODO: Document
• error
  TODO: Document
• last
  Helper macro for the rebind! macro
• list
  TODO: Document
• rebind
  Rebinds an object so that it is bound to an immutable borrow of crate::gc::Context instead of a mutable borrow. This can release the mutable borrow and allow Context to be used for other things.
• root
  Creates a new root that will be traced during garbage collection. The value returned by this macro is no longer bound to the Context and so can be used outside of the Context’s lifetime. The root is tied to the stack, and will be unrooted when it goes out of scope.