fix: collections matchers should display type of expectation

[blgm]

The collections matchers ConsistOf() and ContainElements() are often
used with collections of consistent type, for example:

Expect(t).To(ConsistOf(1, 2, 3))

When this fails, it's more helpful to report:

...to contain elements <[]int | len:3, cap:3>: [1, 2, 3]

Than the previous behavior:

...to contain elements <[]interface {} | len:3, cap:3>: [1, 2, 3]

By using []interface{} only where there's no other option we make it
easier to debug issues - especially for types that display identically.

[onsi]

hey @blgm - this looks good to me. ready for me to merge it in (or you can too, of course!)