Fix a bug in the const version of Maybe::get<typename T>(), where an unassigned
Maybe would try to automatically activate an element despite being const.
Remove unused private members from ProcessTrace: BufferAllocator and Version.
Add seec::trace::ProcessState::getContainingMemoryArea(uintptr_t) const, to get
 the area of memory that contains a given address (if any). Currently this
doesn't support "known" readable/writable regions, because these are not
recorded in the trace.

This also required adding seec::trace::ThreadState::getContainingMemoryArea(),
and seec::trace::FunctionState::getAllocaContaining().

Add some doxygen documentation.