[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/timing.rs

@@ -68,7 +68,6 @@ define_passes! {
 
     regalloc: "Register allocation",
     regalloc_checker: "Register allocation symbolic verification",
-    binemit: "Binary machine code emission",
     layout_renumber: "Layout full renumbering",
 
     canonicalize_nans: "Canonicalization of NaNs",