Consider adding enum metadata to typed map schemas

[davidmorgan]

If we end up using enums a lot, it might be useful to switch the binary format from the current string-based encoding to an index-based one.

To be backwards/forwards compatible, the index does still need to link back to strings. This can be done by including the strings in the typed map schemas.

So: before this change, enum values end up written out as pointers to strings: four bytes. After this change, they end up written as known-sized integers, which be very tightly packed, for example if the enum has three values then they can be packed as two bits. (because four values counting "unknown").