Instance IDs serve no purpose for rendering when they aren't stable from
one frame to the next, and if the index is used in the end anyway, there
is no point in storing a vector of IDs and copying it around.

This commit exposes the id attribute on the instances component,
makes it optional-- only generated by default with the distribute points
on faces node.

Since the string to curves node only added the index as each instance's
ID, I just removed it. This means that it would be necessary to add the
ID data manually if the initial index actually helps (when deleting
only certain characters, for example). I think that's more consistent
with the workflow elsewhere, but I'm not totally set on it.