Support for Duktape coroutines

[bobpepin]

I had to rewrite the context handling code to allow python callbacks to be invoked from Duktape coroutines.
Duktape coroutines share a heap and global object, but each has a distinct context. In the original code, all callbacks were executed inside the context create in the constructor of DuktapeContext(), which led to bugs when using coroutines.
I also removed the threading checks, since the same Duktape context can execute code in different threads, just not at the same time.