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?
Note that to deprecate the function `isblank`, you would delete it from utf8proc.jl (and delete the export), and then edit `deprecated.jl` to add:
```julia
@deprecate isblank(c::Char) c == ' ' || c == '\t'
@deprecate isblank(s::String) all(c -> c == ' ' || c == '\t', s)
```