Better Wasmtime API for embedder (#287)

* Migrate wasm-rust-api code to wasmtime.

wasmtime-api/src/context.rs

@@ -0,0 +1,63 @@
+use std::cell::{RefCell, RefMut};
+use std::hash::{Hash, Hasher};
+use std::rc::Rc;
+
+use wasmtime_jit::{Compiler, Features};
+
+use cranelift_codegen::settings;
+
+#[derive(Clone)]
+pub struct Context {
+    compiler: Rc<RefCell<Compiler>>,
+    features: Features,
+    debug_info: bool,
+}
+
+impl Context {
+    pub fn new(compiler: Compiler, features: Features, debug_info: bool) -> Context {
+        Context {
+            compiler: Rc::new(RefCell::new(compiler)),
+            features,
+            debug_info,
+        }
+    }
+
+    pub fn create(flags: settings::Flags, features: Features, debug_info: bool) -> Context {
+        Context::new(create_compiler(flags), features, debug_info)
+    }
+
+    pub(crate) fn debug_info(&self) -> bool {
+        self.debug_info
+    }
+
+    pub(crate) fn compiler(&mut self) -> RefMut<Compiler> {
+        self.compiler.borrow_mut()
+    }
+}
+
+impl Hash for Context {
+    fn hash<H>(&self, state: &mut H)
+    where
+        H: Hasher,
+    {
+        self.compiler.as_ptr().hash(state)
+    }
+}
+
+impl Eq for Context {}
+
+impl PartialEq for Context {
+    fn eq(&self, other: &Context) -> bool {
+        Rc::ptr_eq(&self.compiler, &other.compiler)
+    }
+}
+
+pub(crate) fn create_compiler(flags: settings::Flags) -> Compiler {
+    let isa = {
+        let isa_builder =
+            cranelift_native::builder().expect("host machine is not a supported target");
+        isa_builder.finish(flags)
+    };
+
+    Compiler::new(isa)
+}