Add Recevier::recv_batch_up_to, try_recv_batch_up_to

[mullr]

This adds a new batch recv variant, aimed at applications where message processing latency is important but batch processing can still be valuable.

I also fixed up the fuzz tests a bit:

• They were failing when trying to allocate a too-large vector, so generated messages are now u16 bytes long.

• 27 was popping up a lot, so the input domain intentionally excludes

  empty messages for the time being.

• try_recv_batch isn't working, so that branch is commented out.

[tokahuke]

Nice! I will review this tomorrow...

[tokahuke]

Approved!