Add function ABI and type layout to StableMIR

This change introduces a new module to StableMIR named `abi` with
information from `rustc_target::abi` and `rustc_abi`, that allow users
to retrieve more low level information required to perform
bit-precise analysis.

The layout of a type can be retrieved via `Ty::layout`, and the instance
ABI can be retrieved via `Instance::fn_abi()`.

To properly handle errors while retrieve layout information, we had
to implement a few layout related traits.

compiler/stable_mir/src/lib.rs

@@ -33,6 +33,7 @@ use crate::mir::Body;
 use crate::mir::Mutability;
 use crate::ty::{ImplDef, ImplTrait, IndexedVal, Span, TraitDecl, TraitDef, Ty};
 
+pub mod abi;
 #[macro_use]
 pub mod crate_def;
 pub mod compiler_interface;