Support Mid-rule actions

I'd like to use mid-rule actions (i.e. for parsing binding constructs into a nameless representation) similar to [what bison offers](https://www.gnu.org/software/bison/manual/html_node/Using-Mid_002dRule-Actions.html#Using-Mid_002dRule-Actions) and was a little disappointed that happy doesn't seem to support these.

Just want to track this here and ask if there are any alternatives.

The issue names no Happy files, tests, or entry points. Start by reading the linked Bison documentation and inspecting Happy's existing action syntax. Done is not defined; clarification is needed on the intended interface, alternatives, implementation scope, and tests for binding constructs.

Written by the indexing model from the issue text.