Introduce visitor bind

[TobiasNx]

from Catmandu Cheat Sheet:

visitor([path:JSONPath])

Execute all fixes in the context of every
element in the data. This fix will set
special context variables:
scalar - for every scalar value found
array - for every array value found
hash - for every hash value found
key - the field name on which the scalar
array or hash is found

# upcast every ‘name’ field in the record
do visitor()
	if all_equal(key,name)
		upcase(scalar)
	end
end

This would make #162 obsolete and would be a more generic approach.l