Replace g3_with -> local()

[lentinj]

To be able to split of formula_utils, we need to have something that's actual R in place of g3_with.

local() is the obvious candidate, replacing g3_with(var1 := val1, var2 := val2, { stock__num <- var1 * var2 }) with local({var1 <- val1 ; var2 <- val2 ; stock__num <<- var1 * var2}).

However, this isn't brilliant since assignments to stock__num outside of local() will still have to use <-. Remembering which to use sounds painful.

There's also a performance concern about all the extra environments, but this could be rewritten out if required.