Fix rlang conflict

[hadley]

library(ellmer)
#> Warning: replacing previous import 'S7:::=' by 'rlang:::=' when loading
#> 'ellmer'

[hadley]

Moved to rlang

[hadley]

Re-opening, because we can't easily remove this from rlang as many downstream packages rely on it to avoid this note:

* checking R code for possible problems ... NOTE
f: no visible global function definition for ':='
Undefined global functions or variables:
  :=