seal: Add benchmarks for dispatchables (#6715)

* seal: Fix syntax that confuses rust-analyzer

* seal: Add benchmarks for Dispatchables

These are only the benchmarks for the dispatchables of
the pallet. Those are not listed in the Schedule because
we do not want to pull the Schedule from storage before
dispatching.

This OK because those costs are not related to actual contract
execution. Those costs (instruction costs, ext_* costs) will
be benchmarked seperatly and entered into the default Schedule.

* seal: Add a maximum code size

* Fix comments from review

* Removed SEED constant