Module self types

[soutaro]

This PR is to update syntax to specify self type constraint on modules.

The current syntax allows arbitrary types to be there.

module M : _Each[Integer, String]       # This is okay
end

module N : _Each[Integer, String] | Integer     # What does this mean???
end

This patch introduces a new syntax construct to limit the types to be there.

module X : _Each[Integer, String], _ToInt, _ToStr
end

This allows making having multiple module declarations more natural. The following has two module declarations for X and the self type constraint means the same with the above.

module X : _Each[Integer, String], _ToInt
end

module X : _ToStr
end