feat(library): add if_then_else Lean/C++ interface

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(kernel): expose imported predicate

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/simplifier): add to_ceqs function that converts a theorem into a sequence of conditional equations

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(build): C++ module dependency problem, and style

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