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/environ/src/module.rs

@@ -6,7 +6,6 @@ use cranelift_wasm::{
     DefinedFuncIndex, DefinedGlobalIndex, DefinedMemoryIndex, DefinedTableIndex, FuncIndex, Global,
     GlobalIndex, Memory, MemoryIndex, SignatureIndex, Table, TableIndex,
 };
-use std::cmp;
 use std::collections::HashMap;
 use std::string::String;
 use std::vec::Vec;