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`.

tests/wast_testsuites.rs

@@ -1,11 +1,13 @@
 extern crate cranelift_codegen;
 extern crate cranelift_native;
+extern crate wasmtime_jit;
 extern crate wasmtime_wast;
 
 use cranelift_codegen::isa;
 use cranelift_codegen::settings;
 use cranelift_codegen::settings::Configurable;
 use std::path::Path;
+use wasmtime_jit::Compiler;
 use wasmtime_wast::WastContext;
 
 include!(concat!(env!("OUT_DIR"), "/wast_testsuite_tests.rs"));