Replace promise-operators with self-evaluating formulas

[lionel-]

Includes #16 and #17.

with this patch instead of:

foo <- "bar"
var1 <- ~foo
var2 <- local({ foo <- "baz"; ~foo })
f_quote(list(!!var1, !!var2))
#> ~list(foo, `_P`(~foo))

we have:

f_quote(list(!!var1, !!var2))
#> ~list(foo, ~foo)

This should make things much more readable when debugging capturing code. The formulas self-evaluate within f_eval():

f_eval(f)
#> [[1]]
#> [1] "bar"
#>
#> [[2]]
#> [1] "baz"