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?
@stevengj: Sure.  I had planned to doing that as a separate PR, but will add it in.  
@nalimilan: You are correct that it stops after finding a `false`, but it is nonetheless slower than a simple loop.  

Given an array of random UTF8Strings defined as below, the islower(String) using all() took about 3x as long as the version using a for loop.
```
s8long = lowercase(randstring(1500))
s8long = replace(s8long,['1','2','3','4','5'], 'δ')  
s8long = replace(s8long,['6','7','8','9','a'], 'ϕ') 

N=1000_000
SZ=1400
rs=UTF8String[]

for i in 1:N
    pos=rand(1:SZ)
    s= s8long[1:pos]*"X" 
    push!(rs, s)
end
```