feat: move filters into a sub-package

[Stebalien]

These really don't belong in the root package.

[raulk]

A package containing a single component introduces complexity for no reason. In idiomatic Go, filters.Filter is also a smell that the package shouldn’t exist.

[Stebalien]

I agree that single-type packages are suboptimal, but it doesn't belong in the root package. A multiaddr set might belong in the root package, but this filter type is highly specialized (special IPNet functions, filter actions, etc.). Remember, this package has implementations in many languages and needs to present a cohesive minimal interface.

How about a compromise:

1. Move go-multiaddr-net into go-multiaddr/net.
2. Move this filter package into go-multiaddr/net.

[raulk]

@Stebalien Deal taken! That simplifies our codebase even further. Is this something you plan on tackling?

[Stebalien]

Yeah. It's been annoying me for a while... I wonder if I can fold go-multiaddr-dns into the deal.

[raulk]

@Stebalien Nowwwww we're talking :-)

[Stebalien]

Replaced with #127.