Ir module linking

[Y-Nak]

This PR adds an IR-level module linking support.

Also added some changes below:

• Remove ValueVisitable and add Visitable/VisitableMut and Visitor/VisitorMut, #[inst(value)] attribute is no longer required to define an instruction
• Require ; at the end of the global declaration for consistency
• Omit -> unit when dumping a function signature in text format.
• Remove the sig field from Function because of meaningless duplication(and only ModuleCtx owns it now).

[Y-Nak]

Maybe, it'd be better to move module_linker to ir crate(it's trivial, though). Do you have any thoughts? @sbillig

[Y-Nak]

@sbillig I moved module_linkerto ir crate.

[sbillig]

👍