[nfc] cast inputs to ctype.h 'is*' functions

[nfc] cast inputs to ctype.h 'is*' functions

The ctype.h `is*` functions are typically implemented as macros using a lookup
table. This is fine, but on Cygwin the compiler is somewhat picky about this:

  lib/ast/fmtesc.c:116:59: warning: array subscript has type char
    [-Wchar-subscripts]
    116 |                           (c == '#' && (b == f || isspace(*(b - 1)))
        |                                                           ^~~~~~~~

This change squashes these warnings.