This changes the behaviour of 'auto' implicits so that by default they
return the first result, rather than checking for unique results. This
is consistent with Idris 1. However, we still want to check for
uniqueness somtimes (for example, with interface search, which should
reject overlapping results) so the 'uniqueSearch' option means that any
auto implicit search for the type should check uniqueness of results.
Fixes#169
