Stop relying on undefined behaviour

[bodil]

smartstring now implements its own boxed string type rather than deferring directly to String, so it no longer makes assumptions it shouldn't be making about the layout of the String struct. This also allows us to organise the boxed struct in a way that will let us rely only on our basic assumption that heap memory is word aligned on both big and little endian architectures. The most immediate consequence of this is that smartstring will now compile on 32-bit big endian architectures such as mips.

Closes #4 #8 #14