The gain, given the "real sites benchmark" below, is obvious:
```
benchmark old ns/op new ns/op delta
BenchmarkHugo-4 14497594101 13084156335 -9.75%
benchmark old allocs new allocs delta
BenchmarkHugo-4 57404335 48282002 -15.89%
benchmark old bytes new bytes delta
BenchmarkHugo-4 9933505624 9721984424 -2.13%
```
Fixes#2495
This avoids having to execute these expensive operations for sites not using these values.
This commit sums up a set of wordcounting and autosummary related performance improvements.
The effect of these kind of depends on what features your site use, but a benchmark from 4 Hugo sites in the wild shows promise:
```
benchmark old ns/op new ns/op delta
BenchmarkHugo-4 21293005843 20032857342 -5.92%
benchmark old allocs new allocs delta
BenchmarkHugo-4 65290922 65186032 -0.16%
benchmark old bytes new bytes delta
BenchmarkHugo-4 9771213416 9681866464 -0.91%
```
Closes#2378
The custom sort functions used from the templates had some subtle data race- and related issues,
especially when used in the single page template.
This commit fixes this by making copies and protect the read and writes with a RWMutex.
The results are cached (it will typically be invoked *number of pages* times with exactly the same data).
This is, not surprisingly, also faster:
```
benchmark old ns/op new ns/op delta
BenchmarkSortByWeightAndReverse 14228 491 -96.55%
benchmark old allocs new allocs delta
BenchmarkSortByWeightAndReverse 1 0 -100.00%
benchmark old bytes new bytes delta
BenchmarkSortByWeightAndReverse 32 0 -100.00%
```
Fixes#1293
