[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/isa/s390x/mod.rs

@@ -7,7 +7,7 @@ use crate::isa::s390x::settings as s390x_settings;
 use crate::isa::unwind::systemv::RegisterMappingError;
 use crate::isa::{Builder as IsaBuilder, TargetIsa};
 use crate::machinst::{
-    compile, MachCompileResult, MachTextSectionBuilder, Reg, TextSectionBuilder, VCode,
+    compile, CompiledCode, MachTextSectionBuilder, Reg, TextSectionBuilder, VCode,
 };
 use crate::result::CodegenResult;
 use crate::settings as shared_settings;
@@ -64,11 +64,7 @@ impl S390xBackend {
 }
 
 impl TargetIsa for S390xBackend {
-    fn compile_function(
-        &self,
-        func: &Function,
-        want_disasm: bool,
-    ) -> CodegenResult<MachCompileResult> {
+    fn compile_function(&self, func: &Function, want_disasm: bool) -> CodegenResult<CompiledCode> {
         let flags = self.flags();
         let (vcode, regalloc_result) = self.compile_vcode(func, flags.clone())?;
 
@@ -83,7 +79,7 @@ impl TargetIsa for S390xBackend {
             log::debug!("disassembly:\n{}", disasm);
         }
 
-        Ok(MachCompileResult {
+        Ok(CompiledCode {
             buffer,
             frame_size,
             disasm: emit_result.disasm,
@@ -127,7 +123,7 @@ impl TargetIsa for S390xBackend {
     #[cfg(feature = "unwind")]
     fn emit_unwind_info(
         &self,
-        result: &MachCompileResult,
+        result: &CompiledCode,
         kind: crate::machinst::UnwindInfoKind,
     ) -> CodegenResult<Option<crate::isa::unwind::UnwindInfo>> {
         use crate::isa::unwind::UnwindInfo;