Add query form read/write methods

[kasei]

Add methods similar to URI->query_form that allow accessing and setting query form parameters on an IRI. For the setter methods, consider returning new IRI objects instead of mutating the existing object:

my $iri = IRI->new('http://example.org/?name=test');
say $iri->query_form('name'); # test
my $iri2 = $iri->query_form('name' => 'hello');
say $iri2->as_string; # http://example.org/?name=hello

[kasei]

Decided to implement only a query form accessor and a constructor method to create a new IRI instance (leaving each IRI instance immutable).