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/libcalls.rs

@@ -1,6 +1,6 @@
-//! Runtime library calls. Note that the JIT may sometimes perform these inline
-//! rather than calling them, particularly when CPUs have special instructions
-//! which compute them directly.
+//! Runtime library calls. Note that wasm compilers may sometimes perform these
+//! inline rather than calling them, particularly when CPUs have special
+//! instructions which compute them directly.
 
 use cranelift_wasm::{DefinedMemoryIndex, MemoryIndex};
 use vmcontext::VMContext;