Move String functions upcase, downcase, capitalize, camelcase, lstrip, rstrip, strip, chop, chomp from stdlib to puppet

Move String functions upcase, downcase, capitalize, camelcase, lstrip, rstrip, strip, chop, chomp from stdlib to puppet and update them to the 4.x function API. The new implementations must be backwards compatible but may add new features.

The length() function was added in PUP-8497 to override the function with the same name in stdlib.
The size() function was however not moved and should be moved in this ticket since the 3.x stdlib implementation is broken for strings that can be converted to Float (it errors).