refactor(frontends/lean/elaborator): delete old_elaborator, and create frontend_elaborator class that will be based on library/elaborator/elaborator

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

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): hook new elaborator in the default frontend

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(frontends/lean/elaborator): fix bugs and adjust tests

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(tests/lean): adjust tests

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