Refactoring: Break down very large table/__init__.py module

[sungwy]

The table/__init__.py module has grown to be extremely large at ~4500 lines, and often leads to issues in searching code on Github. This PR refactors the table/__init__.py module into smaller modules:

table
└__init__.py (Table, Transaction)
└ inspect.py (InspectTable)
└ update
    └__init__.py (TableMetadataUpdate, TableUpdate, TableRequirement)
    └ schema.py (UpdateSchema, _ApplyChanges, PartnerIdByNameAccessor, UnionByNameVisitor)
    └ snapshot.py (UpdateSnapshot, ManageSnapshots, _SnapshotProducer, DeleteFiles, AppendFiles, OverwriteFiles, _ManifestMergeManager,)
    └ spec.py (UpdateSpec)

[kevinjqliu]

makes sense to me, thanks for working on this!