Fifo Remaining Space function + Test File

[MichaelMohn]

I added a "space" function in the fifo library that returns how much room is available in the fifo. I also created a test for fifo.h

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[kartikr27]

Maybe represent 127 as a variable for readability?

[tiandahuang]

this doesn't return how much space is actually left in the fifo:
please consult this for more information on the use of the get and put pointers.
https://en.wikipedia.org/wiki/Circular_buffer