Consider doing missing value propagation before comparison check in iv()

[DavisVaughan]

Also affects iv_diff()

We probably need to do this because vec_compare() can "early exit" if it can determine the comparison value early on. This can cause some confusing errors since we normally say that incomplete rows are going to be propagated as missing rows in the result, and I think that should take precedence over the comparison.

library(ivs)
library(vctrs)

# Can determine comparison early on
start <- data_frame(x = 1, y = NA)
start
#>   x  y
#> 1 1 NA

end <- data_frame(x = 0, y = 2)
end
#>   x y
#> 1 0 2

iv(start, end)
#> Error in `iv()`:
#> ! `start` must be less than `end`.
#> ℹ `start` is not less than `end` at locations: `c(1)`.

#> Backtrace:
#>     ▆
#>  1. └─ivs::iv(start, end)
#>  2.   └─rlang::abort(message) at ivs/R/iv.R:184:4

iv_diff(vec_c(start, end))
#> Error in `iv()` at ivs/R/diff.R:100:2:
#> ! `start` must be less than `end`.
#> ℹ `start` is not less than `end` at locations: `c(1)`.

#> Backtrace:
#>     ▆
#>  1. └─ivs::iv_diff(vec_c(start, end))
#>  2.   └─ivs::iv(start, end) at ivs/R/diff.R:100:2
#>  3.     └─rlang::abort(message) at ivs/R/iv.R:184:4

# Can't determine comparison because of `NA`
start <- data_frame(x = NA, y = 3)
start
#>    x y
#> 1 NA 3

end <- data_frame(x = 0, y = 2)
end
#>   x y
#> 1 0 2

iv(start, end)
#> <iv<data.frame<
#>   x: double
#>   y: double
#> >>[1]>
#> [1] [NA, NA)

iv_diff(vec_c(start, end))
#> <iv<data.frame<
#>   x: double
#>   y: double
#> >>[1]>
#> [1] [NA, NA)

^{Created on 2022-10-09 with reprex v2.0.2.9000}

[DavisVaughan]

We want incompleteness to be handled first because if you switch these around to something like this:

start <- data_frame(x = 0, y = 2)
end <- data_frame(x = 1, y = NA)

then the comparison can also be made early, it looks like start < end, but it STILL doesn't matter in the end because the incompleteness check would kick in and result in a missing interval.

That means it should be clear that we want to apply this rule symmetrically, meaning that incompleteness should be done up front before any actual comparisons.