`getVersion()` is very slow due to `versionInfo()`

Investigation of https://github.com/r-lib/lifecycle/issues/167 reveals that getVersion() is much slower than it needs to be, due to going through versionInfo().

The versionInfo() helper is massively weighed down by having to call readCitationFile() to fill out the $citation slot, but you don't need this at all when getting the version in getVersion().

It would be much faster to have getVersion() just call the C++ function that returns the version number.

library(rstudioapi)

bench::mark(
  package_version(.Call("rs_rstudioVersion", PACKAGE = "(embedding)")),
  getVersion()
)

# A tibble: 2 × 13
  expression            min  median `itr/sec` mem_alloc `gc/sec` n_itr  n_gc total_time result    
  <bch:expr>        <bch:t> <bch:t>     <dbl> <bch:byt>    <dbl> <int> <dbl>   <bch:tm> <list>    
1 "package_version…  14.5µs  15.3µs    59209.        0B    11.8   9998     2      169ms <pckg_vrs>
2 "getVersion()"    376.5µs 391.1µs     2520.    20.6KB     8.30  1215     4      482ms <pckg_vrs>
# ℹ 3 more variables: memory <list>, time <list>, gc <list>

rstudio / rstudioapi

`getVersion()` is very slow due to `versionInfo()` #279