* Renamed `ProxyChannel` -> `BaseProxyChannel` and `SimpleProxyChannel`
-> `ProxyChannel`. It makes the interface more consistent by defining
channels to be associated with a certain src/dst memory region:
`ProxyChannel` as "sema + src/dst + fifo" and `SmChannel` as "sema +
src/dst". BaseProxyChannel is not associated with any memory regions, as
"sema + fifo".
* `ProxyChannelDeviceHandle` now inherits from
`BaseProxyChannelDeviceHandle`, instead of having one as a member.
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