Better support deprecating assignment functions

[hadley]

Why did this used to work?

term::`parameters<-`
#> function (x, value) 
#> {
#>     deprecate_soft("0.1.0", what = "parameters<-", with = "pars<-")
#>     pars(x) <- value
#>     x
#> }
#> <bytecode: 0x7fe02759b3e0>
#> <environment: namespace:term>

^{Created on 2021-01-29 by the reprex package (v0.3.0.9001)}

Also think about

lifecycle::deprecate_warn("0.1.0", what = "`parameters<-`()", with = "`pars<-`()")
#> Warning: ``parameters<-`()` was deprecated in <NA> 0.1.0.
#> Please use ``pars<-`()` instead.

^{Created on 2021-01-29 by the reprex package (v0.3.0.9001)}