Add Function support to LINQ DSL

[pnwpedro]

Problem

We want to expose support for calling User-Defined Functions via LINQ.

Solution

Collections and Indexes return sets whereas Functions can return anything. We could approach this a few ways, but I chose to make the existing QuerySource implementation work for functions. As such, the type declarations for functions are a little counter intuitive. Developers must type the Function based on the element type it returns. For example, if it returns a single value, T in Function<T> must be that type. If it returns an array or set, the T must be the type of the element in the array or set.

E.g. 1: If a function returns a string, the type definition must be Function<string>. The caller may call .Single() to get a single value back, or .ToList() to get a list with a single element.

E.g. 2: If a function returns an array or set of strings, the type definition must still be singular, such as Function<string>, similar to how one declares a Collection<T> or Index<T>.

Changes

• Add DSL for Functions
• Add array support to Page deserializer for cases when an array is returned and the DSL assumes we're interacting with a Page.
• Add single value support to the Page and List deserializers for cases when a function returns a single value but, for example, the user wants to use the LINQ API to get a List.
• Handle objects/primitives and arrays (in addition to sets) in Single/SingleAsync.
• Accept a few misc IDE recommendations

[freels]

Did you address #57 as part of this? Seems like it?

[freels]

LGTM