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 :-)