feat(library/hott/trunc): prove that n-types are (n+1)-types.
feat(hott/trunc): clean up some theorems, prove some basic theorems
refactor(typeof): move typeof to general_notation
doc(algebra): update markdown files
style(library): rename set_category to discrete_category
style(library): add some comments
feat(empty): define negation of types
feat(library): port more of truncation library from Coq HoTT

Everything directly about truncations in the basic truncation library is ported.
Some theorems about other structures still need to be ported.
Also made some minor changes in hott.equiv