partial: remove magic-underscoring of partial template names

This logic has proven to be unnecessarily complex.  The partial
should be about rendering with just locals and no layouts - not
the extra wrinkle of expecting underscored template names.

Doing this greatly simplifies the rendering API needed.  Also it
makes template referencing more straightforward and less magical.
You can keep using the underscored-named-partial convention - just
do it manually.