Implement type expressions as documented.

The most important types are:

• T1 or T2, e.g. for T or error (aka mayfail T)
• T1 and T2, mostly for traits, e.g. number and ordered
• not T, mostly for rules that exclude a given set of type, e.g. ln X:not positive is error "Log of negative"

Internally, the compiler currently generates very basic union types using the | operator.

Ideally, we would be able to implement all this in the library only, once the type interface (issue #11) is properly defined. For example:

type T1 or T2 is
    contains Value is T1.contains Value or T2.contains Value