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/jit/Cargo.toml

@@ -0,0 +1,31 @@
+[package]
+name = "wasmtime-jit"
+version = "0.1.0"
+authors = ["The Cranelift Project Developers"]
+publish = false
+description = "JIT-style execution for WebAsssembly code in Cranelift"
+categories = ["wasm"]
+repository = "https://github.com/CraneStation/wasmtime"
+license = "Apache-2.0 WITH LLVM-exception"
+readme = "README.md"
+
+[dependencies]
+cranelift-codegen = "0.26.0"
+cranelift-entity = "0.26.0"
+cranelift-wasm = "0.26.0"
+cranelift-frontend = "0.26.0"
+wasmtime-environ = { path = "../environ" }
+wasmtime-runtime = { path = "../runtime" }
+region = "1.0.0"
+failure = { version = "0.1.3", default-features = false }
+failure_derive = { version = "0.1.3", default-features = false }
+target-lexicon = "0.2.0"
+
+[features]
+default = ["std"]
+std = ["cranelift-codegen/std", "cranelift-wasm/std"]
+core = ["cranelift-codegen/core", "cranelift-wasm/core", "wasmtime-environ/core"]
+
+[badges]
+maintenance = { status = "experimental" }
+travis-ci = { repository = "CraneStation/wasmtime" }