Performs binary inversion on a character string
──────────────────────────────────────────────────────────────────────────────

 Syntax

        GT_Inv(<cString>) --> <cString2>

 Arguments:

      <cString> is any character string

 Returns:

      <cString2> is a binary inverted version of <cString>

 Description:

      This function takes a character string, turns it into a stream
      of binary digits (each eight 'bits' represents one character
      from the input string, and is it's ASCII value), then inverts
      the 'bits', and finaly turns the stream back into a string of
      ASCII characters.

      Passing the inverted string back to the function will cause it
      to revert to it's initial state.

 Examples:

      ? GT_Inv('101010')           // '010101'

      ? GT_Inv('SAM')              // '½╜¡'

      ? GT_Inv(GT_Inv('SAM'))      // 'SAM'

 Source: GTMATH08.PRG

 Author:  Philip Ide