renames Host.<:< to Host.isSubType
removes Host.erasure

The concept of erasure is a bit problematic as outlined in
https://github.com/scalameta/scalameta/pull/30#issuecomment-62617205.
How about we remove Type.erasure from our API for the time being
until we know better how we're going to use it?
finishes evicting stubs from the semantic API
merges MacroHost.resources and MacroHost.resource
Merge pull request #58 from xeno-by/topic/hosts

further cleans up the host API