Replace lexical defines with inline functions

@michaelcahill, this is a minor change to replace the WT_LEX_CMP #defines inline functions.  For some reason I can't figure out, the #defines confuse zoom, it claims the comparison functions aren't taking as much time as they are, and inlining them seems to fix that problem.  We might as well be consistent and use inlined functions instead of a mixture of inlined functions and #defines, anyway.

This shouldn't affect performance (and if anything slows down, well, that tells us to convert the existing lexicographic inlined functions to #defines!)

It's a pull request in case you're planning a release, but otherwise, this can be merged as soon as it passes the usual tests.