Removed linking of SenTesting framework.
Required that Stacks and their elements be Equatable.

Implemented Ò==Ó for Lists.

Declared and implemented head() and tail(), necessitating some renaming of properties to avoid collisions. These aren't tested yet, but I want to checkpoint the Equatable work before futzing further.

Added some notes about investigating declaring some operations on Stack as computed properties.
Removed commented-out implementation of Ò==Ó for pairs of Stacks. The full implementation seg faulted the compiler. Filed as rdar://17479335