added tau based commands

[Tux9001]

I noticed there was commands based on the pi, so I quickly went in and added some for the better circle constant, tau.

[codecov]

Codecov Report

Merging #141 into master will decrease coverage by 0.1%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #141      +/-   ##
==========================================
- Coverage   95.97%   95.86%   -0.11%     
==========================================
  Files          17       17              
  Lines        1762     1764       +2     
==========================================
  Hits         1691     1691              
- Misses         71       73       +2

Impacted Files	Coverage Δ
lib/interp/interpreter.ex	98.13% <0%> (-0.34%)	⬇️

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