feat(frontends/lean): add synthesis information only for 'explicit' placeholder

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/standard): add classes for relations
feat(library/hott): begin porting Coq HoTT
refactor(library/standard): integrate hott with standard library
feat(library/standard/hott/path.lean): add theorems and clean up file
feat(library/standard): add sigma types and subtypes, make inhabited constructive
feat(library/standard/logic): add class nonempty
feat(library/standard/classes): add nonempty
fix(tests/lean/run): adjust test to changes in the library

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
chore(build): remove hott library directory, and move hott tests

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
checkpoint