Reflect wire protocol.

The spec uses camel case, but the wire protocol is always the other
way around.
Update stale comment
Turns out we don't need all of env.hpp here.
Remove no longer useful comment.
Limits not necessary here.

`datum_t::drop_literals` never creates an array larger than the datum
it was created from; as a result for the datum to exist in the first
place to call `datum_t::drop_literals`, it must therefore satisfy the
array size limit.  Accordingly `datum_t::drop_literals` does not need
the limit parameter--which means that `datum_t::merge(datum_t)`
doesn't need it either.
Add braces around these, per style.