#211 dtfn squashes id of elements in templates

Tests check
* When ids already exist, we reuse those instead of regenerating new ones
* When ids are not provided, we auto-generate symbols in their place

Fixes
* ``ts_syms`` is rewritten to absorb ids as well as field data
* When ``ids`` are being replaced within the content rendered by ``tfn``, we replace them with the existing ids instead of new ones