This is a simple bit vector object that is not resizable after
the initial allocation but can be of arbitrary size.  It will
keep the bti vector entirely on the stack for vectors 64 bits
or less, and will allocate the vector on the heap for larger
sizes.  The API is uniform regardless of storage location.

This is very basic right now and all the APIs are inline functions,
but it is useful for storing an array of boolean values.