Issue 7444 - Require [] for array copies too

http://d.puremagic.com/issues/show_bug.cgi?id=7444

Deprecate ambiguity between slice assignment and element-wise one.
See `test/runnable/assignable.d`

Requires: <del>https://github.com/D-Programming-Language/druntime/pull/314</del> (merged)
Requires: https://github.com/D-Programming-Language/phobos/pull/960