`str` is missing `join`, `upper` and `lower` attributes

[dankox]

Builtin string type doesn't support join, upper and lower functions/attributes.

# fails with missing attribute
",".join(list) 
"hello world".upper()
"HELLo".lower()

• str.join
• str.upper
• str.lower

[sbinet]

based on the changes from #188 it should be relatively easy to implement upper and lower (assuming the strings.ToUpper and strings.ToLower stdlib functions achieve what str.upper and str.lower are expected to do).

do you want to give it a try ?