Clean up CRAN warnings

[jcheng5]

Fixes CRAN warning:

checking S3 generic/method consistency ... WARNING
to_ring:
  function(x)
to_ring.Line:
  function(line)

to_ring:
  function(x)
to_ring.Polygon:
  function(pgon)

to_multipolygon_list:
  function(x)
to_multipolygon_list.SpatialPolygons:
  function(pgons)

to_multipolygon_list:
  function(x)
to_multipolygon_list.SpatialLines:
  function(lines)

to_multipolygon:
  function(x)
to_multipolygon.Polygons:
  function(pgons)

to_multipolygon:
  function(x)
to_multipolygon.Lines:
  function(lines)

See section ‘Generic functions and methods’ in the ‘Writing R
Extensions’ manual.

PR task list:

• Update NEWS
• Add tests (where appropriate)
  • R code tests: tests/testthat/
  • Visual tests: R/zzz_viztest.R
• Update documentation with devtools::document()

[jcheng5]

@schloerke @gadenbuie This is off the main branch, which contains #837, which we had previously said should wait for sf 1.0-10 to hit CRAN before we submit. So maybe this should be rebased to go off of our v2.1.1 tag? And then we have to remember to merge it into main as well.