Tweak constructors to bless lexicals

(perl 5.16.0) Certain constructors seem to balk at blessing \$_[1] and similar here:

 t/counter.t .. Modification of a read-only value attempted
  ... etc ...

This change tweaks constructors for Counter, Number, and String to always bless a lexical, which results in passing tests here.

modified:   lib/Data/Perl/Counter.pm
	modified:   lib/Data/Perl/Number.pm
	modified:   lib/Data/Perl/String.pm