Fixes to speed up, cross links for userguide

pluck: `array_column()` work faster than `foreach()`.
count: if `$array` is string `instanceof` try autoload\find class for check, new version work faster, also rename second attribute as `$recursive` - it's more comprehensible.
is_assoc: I add notice, but maybe add validation to 0 for the first key? It's expensive and not too much more reliable.