py: Support __len__ of rangetype.

[corona10]

Fixes: #39

[codecov-io]

Codecov Report

Merging #40 into master will increase coverage by 0.46%.
The diff coverage is 84.61%.

@@            Coverage Diff            @@
##           master     #40      +/-   ##
=========================================
+ Coverage   65.24%   65.7%   +0.46%     
=========================================
  Files          56      56              
  Lines       10150   10170      +20     
=========================================
+ Hits         6622    6682      +60     
+ Misses       3066    3019      -47     
- Partials      462     469       +7

Impacted Files	Coverage Δ
py/range.go	75% <84.61%> (+75%)	⬆️
py/arithmetic.go	61.69% <0%> (+1.11%)	⬆️
py/list.go	10.05% <0%> (+1.18%)	⬆️
py/function.go	25.95% <0%> (+1.52%)	⬆️
py/sequence.go	23.65% <0%> (+3.22%)	⬆️

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[corona10]

@ncw PTAL

[ncw]

Very nice - thank you :-)