Disconnects Store state fields from Compiler (#1761)

*  Moves CodeMemory, VMInterrupts and SignatureRegistry from Compiler
*  CompiledModule holds CodeMemory and GdbJitImageRegistration
*  Store keeps track of its JIT code
*  Makes "jit_int.rs" stuff Send+Sync
*  Adds the threads example.

examples/threads.rs

@@ -0,0 +1,70 @@
+// You can execute this example with `cargo run --example threads`
+
+use anyhow::{format_err, Result};
+use std::thread;
+use std::time;
+use wasmtime::*;
+
+const N_THREADS: i32 = 10;
+const N_REPS: i32 = 3;
+
+fn print_message(_: Caller<'_>, args: &[Val], _: &mut [Val]) -> Result<(), Trap> {
+    println!("> Thread {} is running", args[0].unwrap_i32());
+    Ok(())
+}
+
+fn run(engine: &Engine, module: Module, id: i32) -> Result<()> {
+    let store = Store::new(&engine);
+
+    // Create external print functions.
+    println!("Creating callback...");
+    let callback_type = FuncType::new(Box::new([ValType::I32]), Box::new([]));
+    let callback_func = Func::new(&store, callback_type, print_message);
+
+    let id_type = GlobalType::new(ValType::I32, Mutability::Const);
+    let id_global = Global::new(&store, id_type, Val::I32(id))?;
+
+    // Instantiate.
+    println!("Instantiating module...");
+    let instance = Instance::new(&store, &module, &[callback_func.into(), id_global.into()])?;
+
+    // Extract exports.
+    println!("Extracting export...");
+    let g = instance
+        .get_func("run")
+        .ok_or(format_err!("failed to find export `eun`"))?;
+
+    for _ in 0..N_REPS {
+        thread::sleep(time::Duration::from_millis(100));
+        // Call `$run`.
+        drop(g.call(&[])?);
+    }
+
+    Ok(())
+}
+
+fn main() -> Result<()> {
+    println!("Initializing...");
+    let engine = Engine::default();
+
+    // Compile.
+    println!("Compiling module...");
+    let module = Module::from_file(&engine, "examples/threads.wat")?;
+
+    let mut children = Vec::new();
+    for id in 0..N_THREADS {
+        let engine = engine.clone();
+        let module = module.clone();
+        children.push(thread::spawn(move || {
+            run(&engine, module, id).expect("Success");
+        }));
+    }
+
+    for (i, child) in children.into_iter().enumerate() {
+        if let Err(_) = child.join() {
+            println!("Thread #{} errors", i);
+        }
+    }
+
+    Ok(())
+}