Logarithmic counting

[tobia]

Here's an efficient counting macro I came up with while reading the chapter on Counting.

It can count a practically unlimited number of tokens with Rust's default recursion limit, it's faster than Slice length, and it produces a constant number.

I assert_eq'ed it on all numbers from 0 to 50, just to be on the safe side.

Enjoy!

[RReverser]

Wow, this is great idea!