Previously you had to create a special virtual object to place in the
`Buffer`'s metadata. This added a lot of difficulty that is probably
unnecessary.
Instead, just have `Buffer` hold an arbitrary object and some simple
functions to copy and delete it. There may be issues with type safety
across translation units, but we'll deal with that when/if we run into
it.
The number of bits in a `BitField` cannot be directly implied from the
size of the buffer (because the buffer gets padded to the nearest sized
word). Thus, the `BitField stored the number of bits in its own
internals.
Unfortunately, that caused issues when passing the `BitField` data
between it and an `ArrayHandleBitField`. If the `ArrayHandleBitField`
resized itself, the `BitField` would not see the new size because it
ignored the new buffer size.
To get around this problem, `BitField` now declares its own
`BufferMetaData` that stores the number of bits. Now, since the number
of bits is stored in the `Buffer` object, it is sufficient to just share
the `Buffer` to synchronize all of the state.
