feat(frontends/lean): add pretty printer configuration options

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/head_map): a simple indexing datastructure

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): add tactic_hint command

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): allow user to suppress proofs in theorems, and let them be inferred automatically using tactic_hints

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): allow tactic_hints to be applied when class-instance mechanism fails

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