Separate the character processing from the interrupt handler using ring buffer

[mpetch]

Separate the character processing from the interrupt handler using a ring buffer:

• Create ring buffer for the scan codes in keybuff.c and keybuff.h
• Remove the character processing from the interrupt handler keyboard.c by using the new scancode ring buffer
• Move the character processing to new files keyproc.c and keyproc.h which will be called outside of an interrupt. keyboard_flush will flush the contents of the scancode buffer. Calling process_key can be called outside of an interrupt to process the next scancode if one is available. Calling process_keys will process all the keys until the scancode buffer is empty.
• Modify main.c to go into an infinite loop calling process_keys waiting for an interrupt between each call.
• Modify TaskA to call process_keys so that keyboard input continues to be processed while the multitasking code is running
• Other minor fixes.

Note: I didn't fix the bugs related to task switching that only allows TaskA and TaskB to switch.

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Information on ring buffers (circular buffers) can be found on Wikipedia and Stackoverflow.