factor_: Fast Factor Generation
In kevinushey/Kmisc: Kevin Miscellaneous

This function generates factors more quickly, by leveraging fastmatch::fmatch.

1	factor_(x, levels = NULL, labels = levels, na.last = NA)

`x`	An object of atomic type `integer`, `numeric`, `character` or `logical`.
`levels`	An optional character vector of levels. Is coerced to the same type as `x`. By default, we compute the levels as `sort(unique.default(x))`.
`labels`	A set of labels used to rename the levels, if desired.
`na.last`	If `TRUE` and there are missing values, the last level is set as `NA`; otherwise; they are removed.