type convert simplifications

[max-hoffman]

dolt bump here: dolthub/dolt#3757

This expands some Convert methods into smaller helper functions that can be used more precisely:

• avoid interface conversions
• sidestep bound checks that don't apply on the read path unless we are explicitly running a Cast

These are a bit rough and ready, the interfaces and semantic check organization could be formalized, but gives 15% boost on table scan.

[zachmu]

LGTM