feat(frontends/lean): add 'class' command back

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(emacs/lean-syntax.el): highlight "parameter (" properly

fix #122
fix(frontends/lean/pp): bug when pretty printing arrows

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library): remove unnecessary [fact] modifier

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(frontends/lean/pp): add necessary '@' when pp.implicit is true, otherwise produced output will not even type check

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
checkpoint for imenu
chore(emacs/lean-mode): remove lean-tab function
checkpoint: restructuring lean-mode