Handle nontrivial defaults better in coef

[richfitz]

gen <- odin({
  deriv(x) <- 1
  initial(x) <- x0
  x0 <- user(-1 / (2 + 3 * 4))
})

mod <- gen$new()
expect_equal(mod$contents()$x0, -1 / (2 + 3 * 4))

coef(mod)$default_value[[1]]

which gives

[[1]]
[1] "/"

[[2]]
[[2]][[1]]
[1] "-"

[[2]][[2]]
[1] 1


[[3]]
[[3]][[1]]
[1] "("

[[3]][[2]]
[[3]][[2]][[1]]
[1] "+"

[[3]][[2]][[2]]
[1] 2

[[3]][[2]][[3]]
[[3]][[2]][[3]][[1]]
[1] "*"

[[3]][[2]][[3]][[2]]
[1] 3

[[3]][[2]][[3]][[3]]
[1] 4