After this commit, if you want to resolve `layouts/_shortcodes/myshortcode.txt` when rendering HTML content, you need to use the `{{%` shortcode delimiter:
```
{{% myshortcode %}}
```
This should be what people would do anyway, but we have also as part of this improved the error message to inform about what needs to be done.
Note that this is not relevant for partials.
Fixes#13698
We were mistakingly using the templates (e.g. list.html) descriptor to resolve the base template and not the page,
which worked fine in most cases, but not all.
Fixes#13583
````
│ stash.bench │ perf-v146.bench │
│ sec/op │ sec/op vs base │
LookupPartial-10 248.00n ± 0% 14.75n ± 2% -94.05% (p=0.002 n=6)
│ stash.bench │ perf-v146.bench │
│ B/op │ B/op vs base │
LookupPartial-10 48.00 ± 0% 0.00 ± 0% -100.00% (p=0.002 n=6)
│ stash.bench │ perf-v146.bench │
│ allocs/op │ allocs/op vs base │
LookupPartial-10 3.000 ± 0% 0.000 ± 0% -100.00% (p=0.002 n=6)
```
THe speedup above assumes reuse of the same partials over and over again, which I think is not uncommon.
This commits also adds some more lookup benchmarks. The current output of these on my MacBook looks decent:
```
BenchmarkLookupPagesLayout/Single_root-10 3031562 395.5 ns/op 0 B/op 0 allocs/op
BenchmarkLookupPagesLayout/Single_sub_folder-10 2515915 480.9 ns/op 0 B/op 0 allocs/op
BenchmarkLookupPartial-10 84808112 14.13 ns/op 0 B/op 0 allocs/op
BenchmarkLookupShortcode/toplevelpage-10 8111779 148.2 ns/op 0 B/op 0 allocs/op
BenchmarkLookupShortcode/nestedpage-10 8088183 148.6 ns/op 0 B/op 0 allocs/op
```
Note that in the above the partial lookups are cahced, the others not (they are harder to cache because of the page path).
Closes#13571
This commit also
* Unexport all internal state in TemplateInfo.
* Make the dispatcher keys used for passing context.Context into uint8 from string to save memory allocations.
Co-authored-by: Joe Mooring <joe@mooring.com>
Updates #13571
