Refactor the compilation and instantiation pipeline.

wasmtime-execute is now wasmtime-jit. Move `JITCode` and the TargetIsa
into a new `Compiler` type. `InstancePlus` is no more, with trampoline
functionality now handled by `Compiler`.

lib/runtime/src/mmap.rs

@@ -110,7 +110,7 @@ impl Mmap {
         self.ptr
     }
 
-    /// Return the lengthof the allocated memory.
+    /// Return the length of the allocated memory.
     pub fn len(&self) -> usize {
         self.len
     }