Draft: introduce Compact Row Codec

[stevenschlansker]

What does this PR do?

Introduce alternate "compact" row encoding that better uses knowledge of fixed-size types and sacrifices alignment to save space.

Introduce new Builder pattern to avoid explosion of Encoders static methods as more features are added to row format.

Optimizations include:

• struct stores fixed-size fields (e.g. Int128. FixedSizeBinary) inline in fixed-data area without offset + size
• struct of all fixed-sized fields is itself considered fixed-size to store in other struct or array
• struct skips null bitmap if all fields are non-nullable
• struct sorts fields by fixed-size for best-effort (but not guaranteed) alignment
• struct can use less than 8 bytes for small data (int, short, etc)
• struct null bitmap stored at end of struct to borrow alignment padding if possible
• array stores fixed-size fields inline in fixed-data area without offset+size
• array header uses 4 bytes for size (since Collection and array are only int-sized) and leaves remaining 4 bytes for start of null bitmap

Fixups include:

• toString better handles varbinary / fixed-binary (hex dump of first 256 bytes)
• start making Javadoc for row format

Compromises:

• less alignment could increase access time, but this is opt-in. and I think on modern processors it is not such a big deal.
• increased complexity of offset lookup, try to pre-compute in an array when possible and use StableValue when it is GA

TODO:
[ ] figure out how top-level array and map encoder fit into new builder class

Not compatible with existing row format.

Related issues

Fixes #2337

Does this PR introduce any user-facing change?

New API for new Compact codec. Existing codec unchanged.