Rename fields when generating map encodings

[hannahhoward]

Goals

Allow using keys other than the actual field name when generating map encodings.

Implementation

If a struct field has a tag with key "json", use that as the "name" for the field when generating a map encoder as opposed to the actual field's name. (otherwise just use the name)

This has a number of potential uses:

1. using shorter key names for size concerns
2. being able to rename struct fields without breaking compatibility with old versions.