Convert string integer to binary integer

Convert an ASCII string representation of an integer to a binary integer value.

- Convenient when the user is required to input integer values from the keyboard.

- Ideas from https://ia600404.us.archive.org/3/items/Compute_s_Machine_Language_Routines_for_the_Commodore_64_and_128/Compute_s_Machine_Language_Routines_for_the_Commodore_64_and_128.pdf (page 133)