[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/codegen/src/lib.rs

@@ -114,7 +114,7 @@ mod value_label;
 #[cfg(feature = "souper-harvest")]
 mod souper_harvest;
 
-pub use crate::result::{CodegenError, CodegenResult};
+pub use crate::result::{CodegenError, CodegenResult, CompileError};
 
 /// Even when trace logging is disabled, the trace macro has a significant performance cost so we
 /// disable it by default.