table: Improve mangling of JSON field names

table: Improve mangling of JSON field names

First, when displaying JSON we convert dashes into underscores. We want
to avoid using dashes in JSON field names in new code, because some
JSON parsers don't support dashes very well.

Second, we make the first character of every word lower-case. This
better matches our JSON field name style, and makes the automatic
JSON name mangling a lot more useful for vertical tables, where fields
are given a display name. For example, "Foo Bar" would be converted into
"foo_bar" instead of "Foo_Bar", which much better matches our style.
We don't make the whole string lowercase to support cases like:
"fooBar" should stay as "fooBar".

Some situations don't behave quite perfectly, such as "Foo BarBaz" gets
converted into "foo_barBaz", or all-caps headings get mangled
incorrectly. In these situations, the JSON field should be overridden
manually. In most cases, or at least more cases than before, this
heuristic does good enough.