Initial implementation of interface conjunctions (#1691)

The basic feature here is the ability to use the `&` operator to produce the conjunction/intersection of two interfaces. That is, you can have interfaces:

    interface IFirst { int getFirst(); }
    interface ISecond { int getSecoond(); }

and if you need a generic function where the type parameter `T` must conform to *both* of these interfaces, you express that by constraining the parameter to the intersection of the interfaces:

    void someFunction<T : IFirst & ISecond>(T value) { ... }

Without this feature, the main alternative an application would have is to define an intermediate interface, like:

    interface IBoth : IFirst, ISecond {}

Forcing users to deal with an intermediate interface creates more work for type authors (they need to remember to inherit from the right combined interface(s)), or for `extension` authors (when you add `ISecond` to a type that used to just support `IFirst`, you had better also add `IBoth`). In the worst case, a family of N related "leaf" interfaces would give rise to an exponential number of intermediate interfaces to represnt the possible combinations.

A conjunction like `IFirst & ISecond` is officially its own type, and can be used to declare a type alias:

    typealias IBoth = IFirst & ISecond;

This change only includes the first pass of work on this feature, so there are several caveats to be aware of:

* Using a conjunction as part of an inheritance clause is not yet supported (e.g., `struct X : IFirst & ISecond`). This is true even if the conjunction was introduced by an intermediate `typealias`

* The `&` syntax introduced here is only parsed in places where only a type (not an expression) is possible. This means you cannot do things like cast to a conjunction with `(IFirst & ISecond)(someValue)`.

* This work *should* apply to conjunctions of more than two interfaces (like `IA & IB & IC`) but that has not yet been tested

* In the long run it may be sensible to allow conjunctions that use concrete types, but we really ought to have the semantic checking logic rule that out for now.

* During testing, I encountered compiler crashes when trying to use this feature together with `property` declarations. Further investigation and debugging is called for.

* The handling of conjunction types is currently incomplete, in that there are many equivalences the compiler does not yet understand. For example, it is clear that `IA & IB` is equivalent to `IB & IA`, but the compiler currently does not understand this and will treat them as different types. A deeper implementation approach is called for.

* Conjunctions are currently only supported for generic type parameter constraints, when performing full specialization. Use of conjunctions for existential-type value parameters or with dynamic dispatch is not yet supported.

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