v2.7

This is a stable-ish release. No further breaking changes are currently
contemplated for the 2.* release series.

Breaking changes:

• bashy-core:
  • arg-processor:
    • Tightened up syntax for passing multi-value arguments.
    • Reworked --enum to be a multi-value option in the same way that the
      system lets clients define them. Most notably, it's now --enum[]=
      instead of --enum=.
    • Changed how --filter functions/code are called, so that they no longer
      run in a subshell, which required changing how the client filter code is
      defined. TLDR: Instead of printing replacement values, they now need to
      call replace-value.

Other notable changes:

• bashy-core:
  • arg-processor:
    • --filter now supports {...} to specify a code snippet, just like
      --call already does.
    • --eval values can be specified using the same multi-value syntax used by
      the rest of the system.
  • misc:
    • Made vals more conservative in its output.
    • New function set-array-from-vals, which is (approximately) the reverse
      action of vals.