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.wat

@@ -0,0 +1,5 @@
+(module
+    (func $message (import "" "hello") (param i32))
+    (global $id (import "" "id") i32)
+    (func (export "run") (call $message (global.get $id)))
+)