trunc_iff_isequiv_truncation via modalities
comments about the reason for repleteness
identity modality

Conflicts:

	theories/Modality.v
	theories/ReflectiveSubuniverse.v
	theories/hit/Truncations.v
eliminate funext (mostly) in ReflectiveSubuniverse.v
eliminate funext (mostly) in Modality.v
update Truncations.v and PropositionalFracture.v for no funext
comments
add unicode notation for identity_modality
simpl rewrite
Merge pull request #507 from mikeshulman/rsu-without-funext

Merging as per @mikeshulman 's suggestion.

eliminate funext (mostly) from reflective subuniverses
Exercises 3.4 and 3.5
Better solution to Exc. 3.4
Better solution to Exc. 3.4