[SERIALIZE] Make serialize uniform and usable

[blythed]

There are several ways in which serialization is not the same in several places:

• Different between SQL & MongoDB (Schema vs inline).
• Different between how users would "create" a serialized version from scratch _leaves.
• The way queries are serialized isn't the same as other classes.

Serialization is also not "usable":

• The nested dict isn't sustainable for users.
• Separation of cls and module unnecessary.

This should be made symmetric and usable.

• [SERIALIZE] Implement ._deep_flat_encode for all leafs #2028
• [SERIALIZE] Replace .encode with ._deep_flat_encode #2029
• [SERIALIZE] Make Schema an option for MongoDB #2030
• [SERIALIZE] Replace cls, module, dict with _path and the rest #2031
• [SERIALIZE] Cleanup extraneous serialization methods #2032
• [SERIALIZE] Auto-infer Schema from data #2042
• [SERIALIZE] Make Document wrapping optional on insert #2043
• [SERIALIZE] Lazy-creation of output tables for ibis to enable auto-inference of output schema #2044
• [SERIALIZE] Fix CI and integration tests for new serialization #2063
• [SERIALIZE] Update docs with new serialize protocol #2073
• [SERIALIZE] Update and check use-cases with new serialize protocol #2074
• [SERIALIZE] Remove deadcode from project #2089