Allow offloading compilation in cranelift-object (#2371)

This commit is a slight refactoring of the `Module` trait and backend in
`cranelift-object`. The goal is to enable parallelization of compilation
when using `cranelift-object`. Currently this is difficult because
`ObjectModule::define_function` requires `&mut self`. This instead
soups up the `define_function_bytes` interface to handle relocations so
compilation can happen externally before defining it in a `Module`. This
also means that `define_function` is now a convenience wrapper around
`define_function_bytes`.

cranelift/module/src/lib.rs

@@ -44,7 +44,7 @@ mod traps;
 pub use crate::data_context::{DataContext, DataDescription, Init};
 pub use crate::module::{
     DataId, FuncId, FuncOrDataId, Linkage, Module, ModuleCompiledFunction, ModuleDeclarations,
-    ModuleError, ModuleResult,
+    ModuleError, ModuleResult, RelocRecord,
 };
 pub use crate::traps::TrapSite;