Easy way to swap a vector out of the heap for non-aliased use

[jorendorff]

This might be a useful hack, at least in the short term.

For example, in Scheme, (write-bytevector vec port) needs to call writer.write(buf) on some buffer. But it's unsafe to create a reference to the actual buffer in vec, because the vector has interior mutability: it could be aliased, and all aliases have write access.

As a hack, write-bytevector could momentarily swap the buffer out of the in-heap Vec, so that it "owns" the buffer and can safely pass it to .write(). The in-heap Vec would be momentarily empty. On success or failure, write-bytevector would swap the buffer back into the heap. Hokey but at least it's not a copy.