Audit public API types for drop impls

[carllerche]

All public API types should be audited to see if they need a Drop impl to maintain correct state.

Current known issues:

• Stream Allow application to configure initial window size, etc #40, Audit public API types for drop impls #95
• Body Dropping a Body implicitly release remaining capacity? #90

[carllerche]

This does not require API breakage. So I will remove it from the 0.1 milestone.

[nox]

Stream, Body, SendResponse, ResponseFuture, SendStream and RecvStream all transitively wrap an OpaqueStreamRef that cleans up after itself in its Drop impl, AFAIK this can be closed now.