flush package:bytebuild

Push the buffer currently being filled onto the chunk list, allocating a new active buffer of the requested size. This is helpful when a small builder is sandwhiched between two large zero-copy builders:

insert bigA <> flush 1 <> word8 0x42 <> insert bigB

Without flush 1, word8 0x42 would see the zero-byte active buffer that insert returned, decide that it needed more space, and allocate a 4080-byte buffer to which only a single byte would be written.