Allow dictionary encoding to support more fixed width types

Currently the dictionary encoding in 2.1 only applies to some 128-bit fixed-width types. I think this was a combination of a few things:

• At the time, we were not bit-packing indices, and so applying dictionary encoding to smaller types was unlikely to be beneficial
• Estimating cardinality on all columns was expensive
• TPC-H data has a lot of 128-bit decimals that benefit from dictionary encoding so there was an outsized (and unrealistic) benchmarking gain to focusing on 128-bit types.

However, we should at least be considering dictionary encoding for 64-bit types and maybe even 32-bit types.