Also cache whole roxygen2 example code, not just top-level expressions

[lorenzwalthert]

Closes #605. styling for fully cached expressions will be a bit slower (factor 65 instead of 80 compared to full styling), but we get large speed gains if roxygen code is not modified but any other token is modified (which is most of the time I think): here with current master:

library(magrittr)
  library(styler)
  text_long <- c(
    "#' Roxygen",
    "#' Comment",
    "#' @examples",
    "#' 1 + 1",
    "#' if (x > 4)",
    "#' bb = 3",
    "#' call(x,y=2)",
    "k <- function() {",
    "  1 + 1",
    "  if (x) {",
    "    k()",
    "  }",
    "}",
    ""
  )
  text_long_styled <- style_text(text_long)
  purrr::map_dbl(1:5,
    function(x) {
      styler:::fresh_testthat_cache()
      text_long_styled <- style_text(text_long)
      text_long_styled[13] <- "    }"
      system.time(style_text(text_long_styled))['elapsed']
  }) %>%
    mean()
#> Using cache testthat at /Users/lorenz/Library/Caches/R/R.cache/styler/testthat.
#> Using cache testthat at /Users/lorenz/Library/Caches/R/R.cache/styler/testthat.
#> Using cache testthat at /Users/lorenz/Library/Caches/R/R.cache/styler/testthat.
#> Using cache testthat at /Users/lorenz/Library/Caches/R/R.cache/styler/testthat.
#> Using cache testthat at /Users/lorenz/Library/Caches/R/R.cache/styler/testthat.
#> [1] 0.281

^{Created on 2020-02-22 by the reprex package (v0.3.0)}

So timing is 0.281 current master and with this PR: 0.1896. The gain is larger the larger the roxgen code example block is.

[codecov-io]

Codecov Report

Merging #607 into master will increase coverage by 0.05%.
The diff coverage is 92.3%.

@@            Coverage Diff             @@
##           master     #607      +/-   ##
==========================================
+ Coverage   90.47%   90.53%   +0.05%     
==========================================
  Files          47       47              
  Lines        2132     2144      +12     
==========================================
+ Hits         1929     1941      +12     
  Misses        203      203

Impacted Files	Coverage Δ
R/addins.R	0% <0%> (ø)	⬆️
R/utils-cache.R	100% <100%> (ø)	⬆️
R/roxygen-examples.R	96.87% <100%> (+0.87%)	⬆️
R/utils.R	82.75% <100%> (+3.44%)	⬆️
R/testing.R	78.5% <92.3%> (+0.07%)	⬆️

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