[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/machinst/mod.rs

@@ -272,7 +272,7 @@ pub trait MachInstEmitState<I: MachInst>: Default + Clone + Debug {
 
 /// The result of a `MachBackend::compile_function()` call. Contains machine
 /// code (as bytes) and a disassembly, if requested.
-pub struct MachCompileResult {
+pub struct CompiledCode {
     /// Machine code.
     pub buffer: MachBufferFinalized,
     /// Size of stack frame, in bytes.
@@ -299,13 +299,18 @@ pub struct MachCompileResult {
     pub bb_edges: Vec<(CodeOffset, CodeOffset)>,
 }
 
-impl MachCompileResult {
+impl CompiledCode {
     /// Get a `CodeInfo` describing section sizes from this compilation result.
     pub fn code_info(&self) -> CodeInfo {
         CodeInfo {
             total_size: self.buffer.total_size(),
         }
     }
+
+    /// Returns a reference to the machine code generated for this function compilation.
+    pub fn code_buffer(&self) -> &[u8] {
+        self.buffer.data()
+    }
 }
 
 /// An object that can be used to create the text section of an executable.