This patch removes (or conditionally removes) the Set method from
portals that are read-only so that IsWritableArrayHandle will work as
expected. The ArrayPortal doxygen has been updated to reflect this.
The remaining exceptions are `ArrayPortalVirtual` and
`ArrayPortalMultiplexer`, since their mutability cannot be determined at
compile time.
Adds a fancy array handle that restricts access to an array to some
window of values. It takes a start offset and a size and represents the
values between that start offset and size past that.
