Add the ability to have a `summary` page variable that overrides
the auto-generated summary. Logic for obtaining summary becomes:
* if summary divider is present in content, use the text above it
* if summary variables is present in page metadata, use that
* auto-generate summary from first _x_ words of the content
Fixes#5800
This is in heavy use in rendering, so this makes a difference:
```bash
benchmark old ns/op new ns/op delta
BenchmarkSiteBuilding/TOML,num_langs=1,num_pages=500,tags_per_page=5,shortcodes,render-4 124551144 107743429 -13.49%
benchmark old allocs new allocs delta
BenchmarkSiteBuilding/TOML,num_langs=1,num_pages=500,tags_per_page=5,shortcodes,render-4 528684 435118 -17.70%
benchmark old bytes new bytes delta
BenchmarkSiteBuilding/TOML,num_langs=1,num_pages=500,tags_per_page=5,shortcodes,render-4 53306848 45147832 -15.31%
```
This commit moves almost all of the template functions into separate
packages under tpl/ and adds a namespace framework. All changes should
be backward compatible for end users, as all existing function names in
the template funcMap are left intact.
Seq and DoArithmatic have been moved out of the helpers package and into
template namespaces.
Most of the tests involved have been refactored, and many new tests have
been written. There's still work to do, but this is a big improvement.
I got a little overzealous and added some new functions along the way:
- strings.Contains
- strings.ContainsAny
- strings.HasSuffix
- strings.TrimPrefix
- strings.TrimSuffix
Documentation is forthcoming.
Fixes#3042
