[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/module/src/module.rs

@@ -9,7 +9,7 @@ use super::HashMap;
 use crate::data_context::DataContext;
 use cranelift_codegen::entity::{entity_impl, PrimaryMap};
 use cranelift_codegen::{binemit, MachReloc};
-use cranelift_codegen::{ir, isa, CodegenError, Context};
+use cranelift_codegen::{ir, isa, CodegenError, CompileError, Context};
 use std::borrow::ToOwned;
 use std::string::String;
 
@@ -192,6 +192,12 @@ pub enum ModuleError {
     Backend(anyhow::Error),
 }
 
+impl<'a> From<CompileError<'a>> for ModuleError {
+    fn from(err: CompileError<'a>) -> Self {
+        Self::Compilation(err.inner)
+    }
+}
+
 // This is manually implementing Error and Display instead of using thiserror to reduce the amount
 // of dependencies used by Cranelift.
 impl std::error::Error for ModuleError {