customizable cache (closes #2176)

[atusy]

This PR allows implementing knit_cache_hook methods which may preprocess objects (e.g., save to an external file) and define custom loaders.

I will add a NEWS item after we agree with the design.

• refactor(cache): use saveRDS/readRDS instead of makeLazyLoadDB/lazyload
  • For migration, cache_save replaces rdb/rdx files with rds file
  • For backward compatibility, cache_load() attempts lazyload() if rdb/rdx files are available
• feat(cache): allow pre/postprocessing cache objects)
  • knit_cache_preprocess preprocesses objects being saved
  • knit_cache_postprocess postprocesses objects being loaded
• feat!(cache): implement knit_cache_hook instead of pre/post-processors
  • Call knit_cache_hook methods on saving cache
    • Methods may save extra files under ${cache_path(h)}__extra directory
    • Methods may return custom loader functions which is saved to ${cache_path(h).rds}

With this PR, we can add some hooks on objects to be cached.
For example, we can use writeLines to save character objects.

```{r}
library(knitr)
registerS3method(
  "knit_cache_hook",
  "character",
  function(x, nm, path) {
    # Cache x as is if it extends character class
    if (!identical(class(x), "character")) {
      return(x)
    }

    # Preprocess data (e.g., save data to an external file)
    # Create external files under the directory of `paste0(path, "__extra")`
    # if knitr should cleanup them on refreshing/cleaning cache
    d <- paste0(path, "__extra")
    dir.create(d, showWarnings = FALSE, recursive = TRUE)
    f <- file.path(d, paste0(nm, '.txt'))
    writeLines(x, f)

    # Return loader function
    # which receives ellipsis for future extentions and has knit_cache_loader class
    structure(function(...) readLines(f), class = 'knit_cache_loader')
  },
  envir = asNamespace("knitr")
)
```

```{r, cache=TRUE}
x <- 'foo bar'
print(x)
```

```{r}
print(x)
```

[atusy]

maybe preprocess and postprocess are not good names... 🤔

[atusy]

cache_purge() and clean_cache() take into account of limited file types/names.
There may be some cases where knitr should remove more files.

The example in the description saved an extra file as a part of cache, which will not be removed by knitr.

registerS3method("knit_cache_preprocess", "data.frame", function(x) {
  write.csv(x, "cache.csv") # NOTE: this file is not removed by `cache$purge()` or `clean_cache()`
  structure("cache.csv", class = "knit_cache_csv")
}, envir = asNamespace("knitr"))

[atusy]

Postprocess is skipped if a package is not loaded.

[atusy]

To solve the above problems, I implemented the knit_cache_hook generic function in place of knit_cache_preprocess and knit_cache_postprocess. See updated description for the details.