Created placeholder files for each module in ExtCore.Collections (except for KeyValuePair, which is trivial).
Moved the implementations of ArrayView.get and ArrayView.set into the Item property of the type extension to enforce bounds-checking.
Implemented some Async-related functions.
Added attributes to auto-open the ExtCore and ExtCore.Collections namespaces, since these'll be used almost always when ExtCore is used in a project.
Implemented Array.contains.
Implemented ArrayView.create.
Modified ArrayView.get and ArrayView.set so they simply alias the Item property of the type extension.
Put a conditional directive around TaggedArray -- we'll soon re-implement it using the F# proto compiler so we can re-use functions implemented for Array instead of having to re-implement them.