Add MiniTuring tutorial.

[sunxd3]

[sunxd3]

This tutorial was written as a personal practice as I tried to understand DynamicPPL. As suggested by @yebai, I made this PR, in case someone thinks it is valuable for future Turing contributors.

The code was verified in a local jupyter notebook setting. I tried to weave the jmd file locally but can't get over the UndefVarError. The stack trace is long, and I don't know if it is helpful or necessary to post the issue. Possible fixes are welcome.

As suggested by @phipsgabler, an example with macroexpand and further explanation may be desirable, but I failed to find a good example for this purpose that is concise and doable with the limited functionality supported by the code. Suggestions on this is very welcomed too.

[yebai]

thanks, @sunxd3!