Dyn method dispatch

[ssyram]

Following #814 , this PR is further split from #762, which includes the part of vtable support for the correct dispatch of virtual methods. Now, calling methods of &dyn Trait will be translated to finding the corresponding function pointer from the vtable (fetched by ptr_metadata operation) and then call the function pointer.

Specifically, calling virtual methods is turned from <dyn Trait as Trait>::method(receiver, args...) to something like:

let vtable = receiver.ptr_metadata;
let method_fn_ptr = vtable.method_index;
method_fn_ptr(receiver, args...)

This is reached by adding a new pass: normalize::transform_dyn_trait_calls.