don't allocate closure variables if the closure doesn't escape

[eliasnaur]

This example,

package main

import (
	"sort"
)

var slice = make([]int, 1000)

func main() {
	closedVar := 10
	closedSlice := slice[10:100]
	// The closure to sort.Find doesn't escape, so
	// neither should the variables closed over.
	sort.Find(len(closedSlice), func(i int) int {
		return closedSlice[i] - closedVar
	})
}

results in:

tinygo build -target pico -print-allocs=. closures.go
/Users/a/proj/tinygo/closures.go:11:2: object allocated on the heap: escapes at line 12
/Users/a/proj/tinygo/closures.go:10:2: object allocated on the heap: escapes at line 12
...

Since the function argument to sort.Find doesn't escape, the variables referenced in the closure shouldn't escape either.

This issue makes it hard to use closures in a GC friendly way, essentially forcing the manual inlining of sort.Find and similar functions. In future, I expect similar issues for Go iterator functions (as anticipated in Go 1.23).