cmd/addchain: templated output generation

[mmcloughlin]

Updates #94

(Transferred from #124 after branch rename.)

[codecov-commenter]

Codecov Report

Merging #127 (dc382fe) into master (49956ae) will increase coverage by 1.19%.
The diff coverage is 85.35%.

@@            Coverage Diff             @@
##           master     #127      +/-   ##
==========================================
+ Coverage   72.81%   74.01%   +1.19%     
==========================================
  Files          30       35       +5     
  Lines        1880     2078     +198     
==========================================
+ Hits         1369     1538     +169     
- Misses        451      474      +23     
- Partials       60       66       +6     

Flag	Coverage Δ
unittests	74.01% <85.35%> (+1.19%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
internal/gen/templates.go	48.00% <48.00%> (ø)
internal/gen/gen.go	66.66% <66.66%> (ø)
internal/gen/functions.go	72.00% <72.00%> (ø)
internal/examples/fp25519/fp.go	100.00% <100.00%> (ø)
internal/examples/fp25519/inv.go	100.00% <100.00%> (ø)

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