Gets rid of the largest set of mutable global state within the core.
This also paves a way for eliminating usages of GetInstance() on the
System class as a follow-up.
Note that no behavioral changes have been made, and this simply extracts
the functionality into a class. This also has the benefit of making
dependencies on the core timing functionality explicit within the
relevant interfaces.
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.
