(math/R-forcats) Updated 0.4.0 to 0.5.1

# forcats 0.5.1

* Re-license as MIT (#277).

* `fct_lump_n()` no longer uses a partial argument name (@malcolmbarrett, #276).

# forcats 0.5.0

* `as_factor()` gains a logical method that always returns a factor with
  levels "FALSE" and "TRUE" (#185).

* `fct_c()`, `fct_collapse()` and `fct_recode()` are now explicitly
  documented as using [dynamic dots](https://rlang.r-lib.org/reference/dyn-dots.html)
  (@labouz, #234).

* `fct_collapse()` now accepts a `other_level` argument, to allow a
   user-specified `Other` level (@gtm19, #194). It now correctly collapses
   factors when `other_level` is not `NULL` (#172), and makes `"Other"` the
   last level (#202) (@gtm19, #172 & #202)

* `fct_count()` no longer converts implicit NAs into explicit NAs (#151).

* `fct_inseq()` behaves more robustly when factor levels aren't all numbers
  (#221).

* `fct_lump()` has been split up into three new functions: `fct_lump_prop()`,
  `fct_lump_n()`, and `fct_lump_lowfreq()`. (@jonocarroll, #167, #142).
  All `fct_lump_()` functions check their inputs more carefully
  (@robinson_es, #169)

* `fct_reorder2()` gains a helper function `first2()`, that sorts `.y` by the
  first value of `.x` (@jtr13).


(mef)