Inefficient encoding of key/values in Layer

Nobody has claimed this yet.

They keys and values use _non-packed_ repeated fields which is inefficient compared to _packed_ repeated fields. In addition the values are each in its own message which needs extra space for the length header. If the values are small integers that could otherwise be encoded in a single byte, this can easily mean they need six times as much space as would be needed otherwise (Value message length + Value message wire type + Value message field number + Layer message wire type + Layer message field number + the value itself).

Unfortunately this can't be changed easily, because the values can be of different types. One option would be to have a `repeated string string_value [packed=true]`, `repeated float float_value [packed=true]`, etc. in the Layer message. But this would mean reordering the keys or having multiple sets of keys for the different types.

Review the vector tile specification's Layer and Value encoding definitions first. Compare the proposed packed per-type fields with the current mixed-type representation, including key/value ordering and compatibility; done requires an agreed encoding design rather than a straightforward edit.

Written by the indexing model from the issue text.