Add support for ratio

[jianlingzhong]

Add support for ratio

Currently, ratio (of two integers) will be converted to a real number during parsing.

Lexing and parsing should work:

❯ build/jank repl   
Bottom of clojure.core
> 4/5
0.8
> 4/5/6
Read error (0 - 3): invalid number
> 4//5
Read error (0 - 4): invalid number
> 4/5/6/7
Read error (0 - 3): invalid number
> 4.4/5
Read error (0 - 3): invalid number
> 5/5.5
Read error (0 - 5): invalid number
> 5/7
0.7142857142857143
> (+ 4 4/5)
4.8
> (+ 4 -4/5)
3.2
> (+ 4 4/-5)
3.2
> (+ 4 -4/-5)
4.8
> (+ -4/5 -4/-5)
0
> 1/0
Read error (0 - 0): Divide by zero