Use a general Eval function for expressions in templates.

[wxiaoguang]

One of the proposals in #23328

This PR introduces a simple expression calculator (templates/eval/eval.go), it can do basic expression calculations.

Many untested template helper functions like Mul Add can be replaced by this new approach.

Then these Add / Mul / percentage / Subtract / DiffStatsWidth could all use this Eval.

And it provides enhancements for Golang templates, and improves readability.

Some examples:

---

• Before: {{Add (Mul $glyph.Row 12) 12}}
• After: {{Eval $glyph.Row "*" 12 "+" 12}}

---

• Before: {{if lt (Add $i 1) (len $.Topics)}}
• After: {{if Eval $i "+" 1 "<" (len $.Topics)}}

FAQ

Why not use an existing expression package?

We need a highly customized expression engine:

• do the calculation on the fly, without pre-compiling
• deal with int/int64/float64 types, to make the result could be used in Golang template.
• make the syntax could be used in the Golang template directly
• do not introduce too much complex or strange syntax, we just need a simple calculator.
• it needs to strictly follow Golang template's behavior, for example, Golang template treats all non-zero values as truth, but many 3rd packages don't do so.

What's the benefit?

• Developers don't need to add more Add/Mul/Sub-like functions, they were getting more and more.
  Now, only one Eval is enough for all cases.
• The new code reads better than old {{Add (Mul $glyph.Row 12) 12}}, the old one isn't familiar to most procedural programming developers (eg, the Golang expression syntax).
• The Eval is fully covered by tests, many old Add/Mul-like functions were never tested.

The performance?

It doesn't use reflect, it doesn't need to parse or compile when used in Golang template, the performance is as fast as native Go template.

Is it too complex? Could it be unstable?

The expression calculator program is a common homework for computer science students, and it's widely used as a teaching and practicing purpose for developers. The algorithm is pretty well-known.

The behavior can be clearly defined, it is stable.

[codecov-commenter]

Codecov Report

Merging #23927 (f1a6b77) into main (f521e88) will decrease coverage by 0.10%.
The diff coverage is 31.71%.

@@            Coverage Diff             @@
##             main   #23927      +/-   ##
==========================================
- Coverage   47.14%   47.04%   -0.10%     
==========================================
  Files        1149     1162      +13     
  Lines      151446   153746    +2300     
==========================================
+ Hits        71397    72336     +939     
- Misses      71611    72891    +1280     
- Partials     8438     8519      +81     

Impacted Files	Coverage Δ
cmd/dump.go	0.66% <0.00%> (-0.01%)	⬇️
cmd/mailer.go	0.00% <0.00%> (ø)
cmd/manager.go	0.00% <0.00%> (ø)
cmd/manager_logging.go	0.00% <0.00%> (ø)
cmd/migrate_storage.go	5.76% <0.00%> (-0.12%)	⬇️
cmd/restore_repo.go	0.00% <0.00%> (ø)
cmd/web.go	0.00% <0.00%> (ø)
models/actions/run.go	1.63% <0.00%> (-0.10%)	⬇️
models/actions/runner.go	1.44% <ø> (ø)
models/packages/package.go	45.45% <0.00%> (-1.13%)	⬇️
... and 65 more

... and 97 files with indirect coverage changes

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