Restore explicit exports of platform API
Restore explicit exports of platform API

It could be argued that the explicit exports of each individual symbol
are actually useful here, since in a way they define the common API that
the various back-ends need to expose. I don't actually have a clear
preference either way; but since this is really orthogonal to the
purpose of the original import change, I'd rather keep the original
behaviour. (It can be changed in a separate PR if desired.)

Using an extra layer of indirection, we can preserve/restore the
individual imports, but without the duplication needed in the initial
version of https://github.com/servo/ipc-channel/pull/108

servo/ipc-channel