io: more hot-rodding for #376

io: more hot-rodding for #376
* Change the interface of read-into to return a slice and an eof boolean separately so the compiler can optimize the slice.
* Add an each-stream-block-slice combinator that behaves like each-block but reuses a preallocated buffer for every iteration.
* Pull some strings in the stream-read-into implementation to further improve type propagation and bounds check elimination.