Unquote-reduce

Instead of:

``` r
x <- c("v1", "v2", "v3")
as.formula(paste("y", paste(vars, collapse="+"), sep="~"))
```

You could write:

``` r
f_new(~ y,  ~ uqr(vars, `+`)))
```

(But that also needs some tweaks to `f_new()`)
