add a "raw" function definition interface to cranelift-module (#1400)

* move trap site definitions into cranelift-module

`cranelift-faerie` and `cranelift-object` already have identical
definitions of structures to represent trap sites.  We might as well
merge them ahead of work to define functions via a raw slice of bytes
with associated traps, which will need some kind of common structure for
representing traps anyway.

* cranelift-module: add `define_function_bytes` interface

This interface is useful when the client needs to precisely specify the
ordering of bytes in a particular function.

* add comment about saving files for `perf`

cranelift/module/src/lib.rs

@@ -35,6 +35,7 @@ use std::collections::{hash_map, HashMap};
 mod backend;
 mod data_context;
 mod module;
+mod traps;
 
 pub use crate::backend::{default_libcall_names, Backend};
 pub use crate::data_context::{DataContext, DataDescription, Init};
@@ -42,6 +43,7 @@ pub use crate::module::{
     DataId, FuncId, FuncOrDataId, Linkage, Module, ModuleError, ModuleFunction, ModuleNamespace,
     ModuleResult,
 };
+pub use crate::traps::TrapSite;
 
 /// Version number of this crate.
 pub const VERSION: &str = env!("CARGO_PKG_VERSION");