Only invoke a given cached partial once

Note that this is backed by a LRU cache (which we soon shall see more usage of), so if you're a heavy user of cached partials it may be evicted and
refreshed if needed. But in most cases every partial is only invoked once.

This commit also adds a timeout (the global `timeout` config option) to make infinite recursion in partials
easier to reason about.

```
name              old time/op    new time/op    delta
IncludeCached-10    8.92ms ± 0%    8.48ms ± 1%   -4.87%  (p=0.016 n=4+5)

name              old alloc/op   new alloc/op   delta
IncludeCached-10    6.65MB ± 0%    5.17MB ± 0%  -22.32%  (p=0.002 n=6+6)

name              old allocs/op  new allocs/op  delta
IncludeCached-10      117k ± 0%       71k ± 0%  -39.44%  (p=0.002 n=6+6)
```

Closes #4086
Updates #9588

metrics/metrics.go

@@ -27,7 +27,7 @@ import (
 
 	"github.com/gohugoio/hugo/common/types"
 	"github.com/gohugoio/hugo/compare"
-	"github.com/gohugoio/hugo/helpers"
+	"github.com/gohugoio/hugo/identity"
 )
 
 // The Provider interface defines an interface for measuring metrics.
@@ -242,7 +242,7 @@ func howSimilar(a, b any) int {
 		return 90
 	}
 
-	h1, h2 := helpers.HashString(a), helpers.HashString(b)
+	h1, h2 := identity.HashString(a), identity.HashString(b)
 	if h1 == h2 {
 		return 100
 	}