Consolidate abstractions and core types into go-libp2p-core

[raulk]

This PR is part of a batch across go-libp2p repos to consolidate abstractions, interfaces and core types into libp2p/go-libp2p-core. See branch consolidate-skeleton there.

Work is being performed in the consolidate-skeleton branch across repos.

In this PR, we:

1. migrate core types out of this repo into go-libp2p-core.
2. ensure backwards compatibility by aliasing all items that were publicly exported.
3. introduce a gomod dependency to go-libp2p-core.

---

Merge strategy: squash and rebase.

[vyzo]

we have more types that need to be aliased -- all the interfaces I would say.

[prestonvanloon]

FYI, this seems to break builds when using go get.

../../libp2p/go-libp2p/p2p/host/basic/basic_host.go:145:32: cannot use h (type *BasicHost) as type host.Host in argument to identify.NewIDService:
	*BasicHost does not implement host.Host (wrong type for Mux method)
		have Mux() *multistream.MultistreamMuxer
		want Mux() "github.com/libp2p/go-libp2p-core/protocol".Switch
../../libp2p/go-libp2p/p2p/host/basic/basic_host.go:172:32: cannot use h (type *BasicHost) as type host.Host in argument to ping.NewPingService:
	*BasicHost does not implement host.Host (wrong type for Mux method)
		have Mux() *multistream.MultistreamMuxer
		want Mux() "github.com/libp2p/go-libp2p-core/protocol".Switch