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/faerie/src/traps.rs

@@ -2,17 +2,7 @@
 //! for every function in the module. This data may be useful at runtime.
 
 use cranelift_codegen::{binemit, ir};
-
-/// Record of the arguments cranelift passes to `TrapSink::trap`
-#[derive(Debug)]
-pub struct FaerieTrapSite {
-    /// 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,
-}
+use cranelift_module::TrapSite;
 
 /// Record of the trap sites for a given function
 #[derive(Debug)]
@@ -22,7 +12,7 @@ pub struct FaerieTrapSink {
     /// Total code size of function
     pub code_size: u32,
     /// All trap sites collected in function
-    pub sites: Vec<FaerieTrapSite>,
+    pub sites: Vec<TrapSite>,
 }
 
 impl FaerieTrapSink {
@@ -34,11 +24,20 @@ impl FaerieTrapSink {
             code_size,
         }
     }
+
+    /// Create a `FaerieTrapSink` pre-populated with `traps`
+    pub fn new_with_sites(name: &str, code_size: u32, traps: Vec<TrapSite>) -> Self {
+        Self {
+            sites: traps,
+            name: name.to_owned(),
+            code_size,
+        }
+    }
 }
 
 impl binemit::TrapSink for FaerieTrapSink {
     fn trap(&mut self, offset: binemit::CodeOffset, srcloc: ir::SourceLoc, code: ir::TrapCode) {
-        self.sites.push(FaerieTrapSite {
+        self.sites.push(TrapSite {
             offset,
             srcloc,
             code,