replace or publish tlsutil

PERF:
Investigate use of buffered io? Optionally? When would you flush?
Use a ring buffer?
Mitigate allocation of windowupdates (pool?)

DOCS:
Doc heartbeat and TypedStreamSession
Doc Accept() returning an error only when the session is closed
Write a more extensive example
Document the muxado wire protocol

TESTING:
tests for heartbeat
tests for typedstream
test for stream exhaustion
test for short read of data frame
tests for window manager
tests for buffer
re-enable stream tests
More session tests
More stream tests
frame/framer tests - return proper error types, hand unknown type frames

### Low priority:
extension: Move high throughput connections to their own connections
window_manager needs to support a deadline
inboundbuffer deadline support
add Reset() stream API?
eliminate unlikely race on s.remoteDebug between handleFrame() and die()
don't send reset if the stream is fully closed
