Linear time list operations

[haampie]

Avoids string concatenation overhead in extend and preextend

"tricks" used:

• Build a local temporary string instead of extending a big string one item at a time (quadratic complexity)
• When prefix not set: use set -- and "$*" with different IFS for fast concatenation, presumably linear complexity.
• When prefix is set: shift the first element, followed by a tight loop (still quadratic complexity).
• Eliminate the array reversal in preextend

In the included benchmark, runtime goes from 100ms to 21ms on my macbook.

[sethrj]

Very nice! Good testing and clever scripting.

[kwryankrattiger]

Looks good, the expected/result are consistent for all of the tests look correct.

I ran all of the test and the benchmark before and after these changes and found one bug in the old preextend command that was double prepending the PREFIX. The speed up for the benchmark as about 4x which is pretty good.

[haampie]

You're right. I think preextend was over-engineered, cause the bug case is never hit (no prefix is every passed).