fix comment typos
comment improvements
consistent use of IsLocal and IsNull local notations
move ooextendable_isnull_fibers to Extensions.v
avoid inexcusably slow typeclass inference
put funext hypotheses explicitly where they occur in Extensions.v
Merge branch 'localization' into modules

Conflicts:
	theories/Modality.v
	theories/ReflectiveSubuniverse.v