1.0.6.56: replace CALL-WITH-DX-FUNCTION with DX-FLET

1.0.6.56: replace CALL-WITH-DX-FUNCTION with DX-FLET

 * Essentially same underlying macroexpansion, but more flexible in
   use, and possibly even easier to read.

 * Use DX-FLET instead of LAMBDAs in our CALL-WITH-FOO expansions
   to avoid consing up closures.