Lighten the script-building framework in `plutus-script-utils`

`plutus-script-utils` provides a convenient, type-safe way to build multi-purpose scripts. However it seems to suffer from inefficiency, which needs investigation and fixing. One first step is to use `template-haskell` to gather cases of purposes where the script does not provide a logic. If that proves insufficient, more research will be needed.

- [ ] make sure TH libraries are available in our dependency tree
- [ ] train in TH
- [ ] build a script-generator that gathers empty purposes using TH