Change CURVE-like sf types to "line"

[yutannihilation]

Fix #3651.

I'm still not sure how these geometries should be handled (or even whether there's a case ggplot2 should handle these or not), but at least COMPOUNDCURVE is converted to lines by sf::st_as_grob(). So, it seems ggplot2 should follow the way.

curve <- st_as_sfc("COMPOUNDCURVE(LINESTRING(0 0, 5 5))")
g <- sf::st_as_grob(curve)
g
#> (gTree[GRID.gTree.2])
is(g[[1]]$children[[1]])
#> [1] "lines"

[thomasp85]

LGTM