Inline strcmp calls in the standard cursor functions

@michaelcahill, I noticed zoom was complaining that strcmp in get/set key was taking up a big chunk of time, and it turns out that clang isn't inlining the calls.  Here's a rework of that, ready to merge if you agree.

There are some minor cleanups along the way:
- always set `fmt` at the start of the function, based on `WT_CURSOR_RAW_OK`
- always check `S` first, then `u`, then `r`