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ARROW-21: Implement a simple in-memory Schema data structure

I also have restored the `nullable` bit to the type metadata only (for the moment mainly to facilitate schema testing / round-trips to Parquet and other media with required/optional distinction) and done some miscellaneous refactoring (`TypeEnum` is renamed to `LogicalType`).

Author: Wes McKinney <wesm@apache.org>

Closes #10 from wesm/ARROW-21 and squashes the following commits:

c770f7d [Wes McKinney] Add simple in-memory Schema data structure. Restore nullable bit to type metadata only. Add "?" to nullable type formatting.