Rewrite interpreter generically (#2323)

* Rewrite interpreter generically

This change re-implements the Cranelift interpreter to use generic values; this makes it possible to do abstract interpretation of Cranelift instructions. In doing so, the interpretation state is extracted from the `Interpreter` structure and is accessed via a `State` trait; this makes it possible to not only more clearly observe the interpreter's state but also to interpret using a dummy state (e.g. `ImmutableRegisterState`). This addition made it possible to implement more of the Cranelift instructions (~70%, ignoring the x86-specific instructions).

* Replace macros with closures

cranelift/interpreter/src/lib.rs

@@ -4,4 +4,8 @@
 
 pub mod environment;
 pub mod frame;
+pub mod instruction;
 pub mod interpreter;
+pub mod state;
+pub mod step;
+pub mod value;