char predicates using libmojibake

see issue #5939 

Please note from string.jl the following:
```
include("utf8proc.jl")
importall .UTF8proc
```

The include was needed to resolve a bootstrap related error.  Is there a preferred means of dealing with this type of issue?
We're just following the Haskell and Go definitions here.  They in turn follow C++ std::iswcntrl().