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
