vim-patch:9.1.0409: too many strlen() calls in the regexp engine (#28857

)

Problem:  too many strlen() calls in the regexp engine
Solution: refactor code to retrieve strlen differently, make use
          of bsearch() for getting the character class
          (John Marriott)

closes: vim/vim#14648

vim/vim@82792db

Cherry-pick keyvalue_T and its comparison functions from patch 9.1.0256.

vim-patch:9.1.0410: warning about uninitialized variable
vim-patch:9.1.0412: typo in regexp_bt.c in DEBUG code

Co-authored-by: John Marriott <basilisk@internode.on.net>