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

helpers/general_test.go

@@ -458,10 +458,3 @@ func BenchmarkUniqueStrings(b *testing.B) {
 		}
 	})
 }
-
-func TestHashString(t *testing.T) {
-	c := qt.New(t)
-
-	c.Assert(HashString("a", "b"), qt.Equals, "2712570657419664240")
-	c.Assert(HashString("ab"), qt.Equals, "590647783936702392")
-}