This commit revises the locking strategy for `partialCached`. We have added a benchmark that may be a little artificial, but it should at least show that we're not losing any performance over this:
```bash
name old time/op new time/op delta
IncludeCached-10 12.2ms ± 2% 11.3ms ± 1% -7.36% (p=0.029 n=4+4)
name old alloc/op new alloc/op delta
IncludeCached-10 7.17MB ± 0% 5.09MB ± 0% -29.00% (p=0.029 n=4+4)
name old allocs/op new allocs/op delta
IncludeCached-10 128k ± 1% 70k ± 0% -45.42% (p=0.029 n=4+4)
```
This commit also revises the template metrics hints logic a little, and add a test for it, which output is currently this:
```bash
cumulative average maximum cache percent cached total
duration duration duration potential cached count count template
---------- -------- -------- --------- ------- ------ ----- --------
163.334µs 163.334µs 163.334µs 0 0 0 1 index.html
23.749µs 5.937µs 19.916µs 25 50 2 4 partials/dynamic1.html
9.625µs 4.812µs 6.75µs 100 50 1 2 partials/static1.html
7.625µs 7.625µs 7.625µs 100 0 0 1 partials/static2.html
```
Some notes:
* The duration now includes the cached invocations (which should be very short)
* A cached template gets executed once before it gets cached, so the "percent cached" will never be 100.
Fixes#4086Fixes#9506
This commit adds support for return values in partials.
This means that you can now do this and similar:
{{ $v := add . 42 }}
{{ return $v }}
Partials without a `return` statement will be rendered as before.
This works for both `partial` and `partialCached`.
Fixes#5783
