Use cstring for C binding

[cdunn2001]

    var s = newString(1024)
    
    
    Run

That creates a buffer of 1025 characters, but only 1024 are legally accessible. 
I.e. 
    
    
    s[1024] = 'Z'
    
    
    Run

is an error: 
    
    
    Error: unhandled exception: index 1024 not in 0 .. 1023 [IndexDefect]
    
    
    Run

It's an important distinction when interacting with C-code because many C 
functions want to read/write that final 0 (using "char*" i.e. "cstring), while 
Nim will write it implicitly at the end of a "string".

That means you (usually) can and should add 1 to "len" when telling C the 
buffer size, e.g. 
    
    
    snprintf(s.cstring, 1025, "Foo bar %s", "Hello");
    
    
    Run