Vector sort

[increscent]

Here's my first pass at the sort! implementation for issue #9

Feel free to improve the lisp code, because I'm not very good at writing lisp yet.

This is the algorithm I used: https://en.wikipedia.org/wiki/Quicksort#Lomuto_partition_scheme
You might be able to make it faster using the Hoare partition scheme. The problem I ran into was that we don't have a > operator. We only have the operator the user gives us, which could be <, in which case we only have < and >=. The workaround I used for the Lomuto partition scheme was to compare the pivot value based on index for equality to get <= to work. I thought I could use the inverse of that to get >, but it didn't work for some reason.

Also, I think lisp_subvector had a bug where it would start at 0 no matter what you put for the start. Maybe my assumptions are wrong though.

[justinmeiners]

Thanks! I'll take a closer look.

All ordering operators are derivable from <:
a > b <=> b < a
a >= b <=> !(a < b)
a <= b <=> !(b < a)

Vector literals can be defined like so: #(1 2 3) so you don't have to use list->vector.

[justinmeiners]

I merged it in and made changes based on it. See #28

Here are my Lisp style recommendations:

• The variables pivot and i defined in the outer let could probably be defined as additional variables on do. For example: (pivot hi pivot) in the do list will initialize pivot to hi and then never modify it.
• I think you should make a separate vector-swap! function. It could even be implemented in C? (This also makes me want to make a swap! macro).