Also, the rsvg_new_SOME_NODE_TYPE () constructor functions now take the
element name.

This lets us:

* Use bsearch() on the sorted array of element names, instead of an
  unordered chain of if (strcmp (...)).  We can replace this later with
  something better like a trie.

* Port things to Rust more easily, since the constructor functions can
  dispatch on their own based on the element name.

* Avoid special cases in the way
  rsvg_new_node_component_transfer_function() and
  rsvg_new_node_light_source() are called; now they dispatch on their
  own based on the element name.