Flash Messages are now lists of strings

[gsteel]

Q	A
Documentation	not-yet
Bugfix	no
BC Break	yes
New Feature	yes
RFC	yes
QA	yes

This patch breaks BC so it'll need to go into 2.x if acceptable.

The main user-facing change is that flash messages are now list<non-empty-string> instead of mixed.

Previously:

$flash->flash('info', 'whatever');
$flash->flash('info', 'other stuff');
// Next Hop
echo $flash->getFlash('info'); // "other stuff"

Now:

$flash->flash('info', 'whatever');
$flash->flash('info', 'other stuff');
// Next Hop
var_dump($flash->getFlash('info')); // ["whatever", "other stuff"]

Tests have been simplified by replacing mocks with concrete instances. Considering that this library only works with Mezzio\Session, there's little point mocking Session and any BC breaks in Session will be discovered here during lock file maintenance.

Docs will need updating which I'll do in separate patch if this one is acceptable