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
4ef9baf5bd