Add R7RS-small BBHW using Guile

[samirose]

Hi and thanks for the project! Heard about it through The Array Cast podcast. I checked out the scheme BBHW versions and decided to contribute a version using R7RS-small and Guile. I deviated a bit from the existing Scheme BBHW implementations to remove mutation of variables.

There are couple of versions:

• 5c5dc2e: resolve the countdown iterations with an immutable do loop
• 17631cf / HEAD: resolve the iterations with a recursive function

I personally prefer the latter version.

Disclaimer: I am not a professional Schemer, it is a hobby language for me.

[c4augustus]

Yep, it now builds Guile and runs BBHW successfully on my macOS