testing/quick: generate more map and slice states

testing/quick: generate more map and slice states

This change adds support in testing/quick to generate maps and slices
in additional states:

  (1.) nil maps

  (2.) nil slices

  (3.) empty slice occupancy: `len(s) == 0 && s != nil`

  (4.) partial slice occupancy: `len(s) < cap(s) && s != nil`

  (5.) full slice occupancy: `len(s) == cap(s) && s != nil`

Prior to this, only #5 was ever generated, thereby not sufficiently
exercising all of the fuzzable code path outcomes.

This change depends on https://go-review.googlesource.com/#/c/17499/.

Change-Id: I9343c475cefbd72ffc5237281826465c25872206
Reviewed-on: https://go-review.googlesource.com/16470
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>