Require bugfixed (and a tad faster) CAG
Fix very subtle but deadly bug in single()

The _select_args result storage introduced in 975b573a did not take
into account that single() operates on the $rs itself, not a fresh
one. As such the first single() arg would be carried around and then
applied to subsequent next() etc iterations. Stop that, as we only
need the attrs in case of collapse => 1 and single() explicitly does
not support this.

While at it do not reuse the cached args at all - while it *appears*
safe, it may very well not be. Leave this for another day
Stop instantiating a cursor on single() use

This minor pessimization snuck in all the way back in 4e9fc3f3.
I am actually not entirely sure whether this isn't a bug - after
all this way Cursor::Cached and friends are bypassed. However
0.08210 has an identical behavior, thus sod it
Unbreak devrel version checks

One needs to supply the literal (string) value of a devrel to VERSION()
otherwise the checks fail. However in this case 5.8 emits annoying warnings.
Abstract the whole insanity in a neatish util function and use that.