cl: support range over function types (Go 1.23)

[xgopilot]

Requested by @xushiwei

This PR adds support for Go 1.23 iterator functions that can be used with the range statement.

Summary

• Added checkIteratorFunc() function in stmt.go to detect iterator function signatures
• Modified getKeyValTypes() to handle *types.Signature for iterator functions
• Added 3 test cases covering all iterator variants

Iterator Function Signatures

Go 1.23 introduced range-over-function with these signatures:

• func(yield func() bool) - 0 values
• func(yield func(V) bool) - 1 value
• func(yield func(K, V) bool) - 2 values (key-value pairs)

Examples

func bar(yield func(v string) bool) {
    yield("Hello")
    yield("World")
}

for v := range bar {
    println(v)  // Output: Hello, World
}

Test Cases

• TestForRangeFunc0 - 0-value iterator (for range foo {})
• TestForRangeFunc1 - 1-value iterator (for v := range bar {})
• TestForRangeFunc2 - 2-value iterator (for k, v := range weekdays {})

All tests pass ✅

Fixes goplus/xgo#2607

Generated with codeagent
Co-authored-by: xushiwei 396972+xushiwei@users.noreply.github.com

[codecov]

Codecov Report

❌ Patch coverage is 57.69231% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.91%. Comparing base (e9785e5) to head (f42e14c).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
stmt.go	57.69%	6 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #586      +/-   ##
==========================================
- Coverage   96.06%   95.91%   -0.15%     
==========================================
  Files          25       25              
  Lines        6855     6881      +26     
==========================================
+ Hits         6585     6600      +15     
- Misses        202      208       +6     
- Partials       68       73       +5     

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