[cranelift] Rejigger the compile API (#4540)

* Move `emit_to_memory` to `MachCompileResult`

This small refactoring makes it clearer to me that emitting to memory
doesn't require anything else from the compilation `Context`. While it's
a trivial change, it's a small public API change that shouldn't cause
too much trouble, and doesn't seem RFC-worthy. Happy to hear different
opinions about this, though!

* hide the MachCompileResult behind a method

* Add a `CompileError` wrapper type that references a `Function`

* Rename MachCompileResult to CompiledCode

* Additionally remove the last unsafe API in cranelift-codegen

cranelift/object/src/backend.rs

@@ -313,11 +313,7 @@ impl Module for ObjectModule {
 
         ctx.compile_and_emit(self.isa(), &mut code)?;
 
-        self.define_function_bytes(
-            func_id,
-            &code,
-            ctx.mach_compile_result.as_ref().unwrap().buffer.relocs(),
-        )
+        self.define_function_bytes(func_id, &code, ctx.compiled_code().unwrap().buffer.relocs())
     }
 
     fn define_function_bytes(