feat(builtin/macros): add obtain-from macro for ExistsElim

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(builtin/Nat): use obtain-from instead of ExistsElim, and use more user-friendly argument order for Induction

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
doc(doc/lean/calc.md): calculational proof documentation

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
doc(doc/lean/calc.md): calculational proof documentation