swift234

[build-script-impl] Optimize setting -> variable name lookups.

 - The current behavior of mapping via `tr` was very slow when called over and
   over for each option. This matters for SR-237 where I would like
   `build-script` to take over the top-level build process control loop, which
   involves executing `build-script-impl` multiple times.

 - This patch optimizes the lookup in two ways:

   1. It does the setting to variable conversion in a single pass, for all
      options at once.

   2. It builds a cache of the setting to variable name conversion (as an
      exploded associate array), and uses that when doing variable assignment.

 - This patch speeds up `build-script-impl --dry-run` by 2x on one test case.