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/imports.rs

@@ -3,7 +3,7 @@ use cranelift_wasm::{FuncIndex, GlobalIndex, MemoryIndex, TableIndex};
 use vmcontext::{VMFunctionImport, VMGlobalImport, VMMemoryImport, VMTableImport};
 
 /// Resolved import pointers.
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 pub struct Imports {
     /// Resolved addresses for imported functions.
     pub functions: BoxedSlice<FuncIndex, VMFunctionImport>,