Add Release() method to capnp.Arena.

[lthibault]

Exports the Release() method from the public Arena interface, to support pooling of arenas. Adds Release() method to SingleSegmentArena, and other private arena types.

[zenhack]

Minor gripe about the docs, but otherwise this looks ok.

[lthibault]

--- FAIL: TestSendDisembargo (0.01s)
    --- FAIL: TestSendDisembargo/SendQueuedResultToCaller (0.01s)
Error:         level1_test.go:285: Received finish message; want disembargo
Error:         level0_test.go:2126: conn error: rpc: send release: send message: context canceled`

This again. Merging.