Patching `BoolArray` always copies (never in-place)

[lwwmanning]

broken in #1742

Because we wrap our Buffer into an Arrow Buffer (which uses Deallocation::Custom under the hood) in order to use BooleanBuffer, we can never get a MutableBuffer back out.

Probably need our own BooleanBuffer type

[gatesn]

Turns out, Arrow boolean buffers aren't in-place mutable anyway...