Places all of the timing-related functionality under the existing Core
namespace to keep things consistent, rather than having the timing
utilities sitting in its own completely separate namespace.
swap.h only needs to be present in the header for the type aliases and
definitions, it's not actually needed in the cpp files though. input.h
is just unused entirely in xpad.h
These classes are non-trivial and are definitely going to be changed in
the future, so we default these to prevent issues with forward
declarations, and to keep the compiler from inlining tear-down code.
