Add variable-length-quantity exercise

[ageron]

I started by using the bitwise operators, and I got stuck with a weird bug: it turned out I was using Num.shiftRightBy instead of Num.shiftRightZfBy, therefore large U32 values would not get smaller and I'd get an infinite loop:

» 4294967295u32 |> Num.shiftRightBy 7 # NOT equivalent to // 128 for large values
4294967295 : U32

» 4294967295u32 |> Num.shiftRightZfBy 7 # always equivalent to // 128
33554431 : U32

[isaacvando]

Looks good!