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/traps.rs

@@ -0,0 +1,14 @@
+//! Defines `TrapSite`.
+
+use cranelift_codegen::{binemit, ir};
+
+/// Record of the arguments cranelift passes to `TrapSink::trap`.
+#[derive(Clone, Debug)]
+pub struct TrapSite {
+    /// Offset into function.
+    pub offset: binemit::CodeOffset,
+    /// Source location given to cranelift.
+    pub srcloc: ir::SourceLoc,
+    /// Trap code, as determined by cranelift.
+    pub code: ir::TrapCode,
+}