Better Wasmtime API for embedder (#287)

* Migrate wasm-rust-api code to wasmtime.

wasmtime-api/src/runtime.rs

@@ -0,0 +1,114 @@
+use std::cell::RefCell;
+use std::collections::HashMap;
+use std::rc::Rc;
+
+use crate::context::{create_compiler, Context};
+
+use cranelift_codegen::settings;
+use wasmtime_jit::Features;
+
+// Runtime Environment
+
+// Configuration
+
+fn default_flags() -> settings::Flags {
+    let flag_builder = settings::builder();
+    settings::Flags::new(flag_builder)
+}
+
+pub struct Config {
+    flags: settings::Flags,
+    features: Features,
+    debug_info: bool,
+}
+
+impl Config {
+    pub fn default() -> Config {
+        Config {
+            debug_info: false,
+            features: Default::default(),
+            flags: default_flags(),
+        }
+    }
+
+    pub fn new(flags: settings::Flags, features: Features, debug_info: bool) -> Config {
+        Config {
+            flags,
+            features,
+            debug_info,
+        }
+    }
+
+    pub(crate) fn debug_info(&self) -> bool {
+        self.debug_info
+    }
+
+    pub(crate) fn flags(&self) -> &settings::Flags {
+        &self.flags
+    }
+
+    pub(crate) fn features(&self) -> &Features {
+        &self.features
+    }
+}
+
+// Engine
+
+pub struct Engine {
+    config: Config,
+}
+
+impl Engine {
+    pub fn new(config: Config) -> Engine {
+        Engine { config }
+    }
+
+    pub fn default() -> Engine {
+        Engine::new(Config::default())
+    }
+
+    pub(crate) fn config(&self) -> &Config {
+        &self.config
+    }
+
+    pub fn create_wasmtime_context(&self) -> wasmtime_jit::Context {
+        let flags = self.config.flags().clone();
+        wasmtime_jit::Context::new(Box::new(create_compiler(flags)))
+    }
+}
+
+// Store
+
+pub struct Store {
+    engine: Rc<RefCell<Engine>>,
+    context: Context,
+    global_exports: Rc<RefCell<HashMap<String, Option<wasmtime_runtime::Export>>>>,
+}
+
+impl Store {
+    pub fn new(engine: Rc<RefCell<Engine>>) -> Store {
+        let flags = engine.borrow().config().flags().clone();
+        let features = engine.borrow().config().features().clone();
+        let debug_info = engine.borrow().config().debug_info();
+        Store {
+            engine,
+            context: Context::create(flags, features, debug_info),
+            global_exports: Rc::new(RefCell::new(HashMap::new())),
+        }
+    }
+
+    pub fn engine(&self) -> &Rc<RefCell<Engine>> {
+        &self.engine
+    }
+
+    pub(crate) fn context(&mut self) -> &mut Context {
+        &mut self.context
+    }
+
+    // Specific to wasmtime: hack to pass memory around to wasi
+    pub fn global_exports(
+        &self,
+    ) -> &Rc<RefCell<HashMap<String, Option<wasmtime_runtime::Export>>>> {
+        &self.global_exports
+    }
+}