Notice: now we use "lang: " prefix when adding language-related commits

It has become too cumbersome to find the issue number when committing some changes about a language kernel. I think this adds significant mental overheads now as we have dozens of language kernels with irregularly ordered issue numbers.

So I'd like to make a little more room for flexibility of issue references. Let's just use a simple prefix like "python: ", "rust: ", etc. when committing changes about a specific language kernel.
Of course, still we should use explicit issue numbers for bugs and new feature sets.
There is no clear-cut rule when to use numbers or prefixes – you may even mix them – but let's have two key principles:
1. **Make the changes recorded and searchable afterwards.**
1. **Git itself is the best tool to record changes with good commit messages and atomic commits.**

For the record of history, here I gathered all lang-related issues:

* Python #1
* R #2
* PHP #4
* NodeJS #5
* Go #9
* Octave #10
* Julia #11
* Haskell #18
* Git #19
* Lua #22
* Shell #31
* TensorFlow + Keras #32
* PyTorch #43
* C #44
* C++ #45
* Java #46
* Rust #48

I left a few issues about languages that are NOT implemented yet, though.