metrics: Detect partialCached candidates

metrics: Detect partialCached candidates

This commit adds a "cache potential" column when running `hugo --templateMetrics --templateMetricsHints`.

This is only calculated when `--templateMetricsHints` is set, as these calculations has an negative effect on the other timings.

This gives a value for partials only, and is a number between 0-100 that indicates if `partial` can be replaced with `partialCached`.

100 means that all execution of the same partial resulted in the same output.

You should do  some manual research before going "all cache".