feat(frontends/lean): parse and pretty print tuples/pairs

This commit also fixes a bug in the type checker when processing dependent pairs.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): support for nary-tuples, improve notation for non-dependent tuples, add support in the elaborator for sigma types

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): parse and pretty print pair/tuple projection operators proj1 and proj2, fix bug in the type checker

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