Investigate/fix new max key size for map stack items

[spencercorwin]

Map stack items now have a max key size. We need to make sure that anywhere that we set key-value pairs in map stack items the key has to be sliced if it's too large. And then all key lookups need to slice the key the same way before doing the lookup. This is handled by the SliceKeyHelper and may have even been done everywhere. But this issue is to check to make sure it's implemented correctly and everywhere.

This also poses a problem for getting the keys from a map with the KEYS opcode. So look into that too.