Allow passing of both positional and named parameters to producer

[vrurg]

In certain circumstances it would be useful to provide the producer with
additional named parameters. For example, I use it to vivify missing
database records. For now the only way for signal that vivification is
allowed is dynamic variables. Whereas it could be done like:

$cache.get(|@pos, :vivify);

The patch is passing all tests. But if this call format is desirable:

$cache.get(@pos)

then get signature should be changed to more explicit slurping. Though to my view bypassing parameters as-is is the most transparent option.