Be more GLib-like, and use

  <namespace>_<type>_from_data()
  <namespace>_<type>_to_data()

Instead of the homebrew "construct" and "serialize", when dealing
with string buffers.

This means:

  • adding json_gobject_from_data() to deprecate
    json_construct_gobject()
  • adding json_gobject_to_data() to deprecate
    json_serialize_gobject()

The json_construct_gobject() function also contains a mistake: it
uses gsize with the special value of -1 meaning "slurp the whole
string", but gsize is an unsigned type. The newly added
json_gobject_from_data() correctly uses gssize instead.