Hash.new with default is fairly common.

Some of these could be zero-allocation with a bit more work, but
this at least removes one array. copy_stream from IO to IO is just
about as close to alloc-free as we can make it.

Frequently used with a frozen literal string and doing no other
allocation. This makes it alloc-free in such cases.

It redispatches to Proc#curry, which currently is in Ruby and
still boxes the arguments, but we're kicking the can down the road
a bit.