Extended String Manipulation Functions for C02

This library contains less commonly used string functions. It is maintained
as a separate set of files so that it can be included only when needed, in
order to reduce program size.

Strings follow all the same rules as specified in the string.h02 library.

Usage: at the beginning of the program use the directives

  #include <stddef.h02>
  #include <string.h02>
  #include <stringx.h02>
  
The following functions are defined:



  n = strspn(&s);     Returns the length of the span of characters at
                      the beginning of destination string set by prior 
                      strdst() call that are present in source string s.

                      This mimics the functionality of the standard C
                      and C++ strspn() function.
                      
                      Note: calls routine strchr(), leaving dstlo and 
                      dsthi pointing to the destination string.

  n = strcsp(&s);     Returns the length of the span of characters at
                      the beginning of destination string set by prior 
                      strdst() call that are not present in source 
                      string s.

                      This mimics the functionality of the standard C
                      and C++ strcspn() function.
                      
                      Note: aliased to the strbrk() function, which calls 
                      strchr(), leaving dstlo and dsthi pointing to the 
                      destination string.

  n = strpbk(&s);     Returns the poisition of the first character in
                      the destination string set by prior strdst() call 
                      that is present in source string s.
                      
                      If no characters in the source string are present
                      in the destination string, returns 255.

                      This mimics the functionality of the standard C
                      and C++ strpbrk() function.
                      
                      Note: calls routine strchr(), leaving dstlo and 
                      dsthi pointing to the destination string.

Note: This library expects the following functions to be defined

  setdst(&s);         Set destination string pointer
  setsrc(&s);         Set source string pointer and initialize index
  strchr(c, &s);      Return position of character in string

along with the zero page pairs
  
  strlo, strhi        Source String Pointer 
  dstlo, dsthi        Destination String Pointer

and the memory locations

  temp0, temp1        Temporary storage

