add stream_input class and build tee_input on it

We will eventually expose a Unicorn::StreamInput object as
"rack.input" for Rack 2.x applications.  StreamInput allows
applications to avoid buffering input to disk, removing the
(potentially expensive) rewindability requirement of Rack 1.x.

TeeInput is also rewritten to build off StreamInput for
simplicity.  The only regression is that TeeInput#rewind forces
us to consume an unconsumed stream before returning, a
negligible price to pay for decreased complexity.

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