feat(library/unifier): add support for choice constraint

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean/builtin_cmds): change notation for marking implicit/cast parameter in sections

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(emacs): modify emacs mode to reflect recent changes

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