Bug with free variables involving characters
See the following session:
Prelude> True:xs where xs free
True : Free1
Prelude> 1:xs where xs free
1 : Free1
Prelude> 'a':xs where xs free
# curry2go/Maincurry2go161214
Maincurry2go161214/Maincurry2go161214.go:15:5: undefined: Prelude
ERROR: Build failed!