Introduce descriptor for multiplied constraints

[muukii]

still under the consideration of the syntax.

a.width(.toSuperview.width, multiplier: 1)

a.width(.to(b).width, multiplier: 1)

a.width(.to(b).width, .exact(10), multiplier: 1)

• [x] Review the syntax
• [x] Add tests

[muukii]

https://github.com/muukii/MondrianLayout/issues/44