Currently the fromObj function takes a JSON object which is then
converted into a list of (String, JSValue) in which we make a lookup.
However, most of the callers of this function call it repeatedly on the
same object, which means we do the object→list conversion repeatedly.

This patch converts it to take directly the list, and converts its
callers to do the conversion themselves (and only once).

While this is not in the hot-path today, it would be if we ever were to
process much data over Luxi (or RAPI), and is a good cleanup in any
case.