wk 0.7.0

• Remove legacy headers that are no longer used by any downstream package
  (#146).
• validate_wk_wkt() now errors for an object that does not inherit from
  'wk_wkt' (#123, #146).
• Added wk_crs_projjson() to get a JSON representation of a CRS object.
  To make lookup possible based on shortcut-style CRS objects (e.g.,
  "EPSG:4326" or 4326), added data objects wk_proj_crs_view and
  wk_proj_crs_json that contain cached versions of rendered PROJJSON
  based on the latest PROJ version (#147).
• Added a wk_crs_proj_definition() method for wk_crs_inherit() (#136,
  #147).
• Conversion to sf now uses the sfc_writer() for all wk classes, making
  conversions faster and fixing at least one issue with conversion of NA
  geometries to sf (#135).
• wk_plot() now plots NULL/NA geometries and mixed geometry types
  more reliably (#142, #143, #149).
• Exported EMPTY geometries to well-known text now include dimension
  (e.g., POINT Z EMPTY) (#141, #150).
• Fixed bug where wk_polygon() doubled some points when the input contained
  closed rings (#134, #151).
• Fixed bug where wk_count() exposed uninitialized values for empty input
  (#139, #153).
• The xy_writer() now opportunistically avoids allocating vectors for Z
  or M values unless they are actually needed (#131, #154).
• Added example WKT for all geometry types and dimensions plus helper
  wk_example() to access them and set various properties (#155).
• Fixes warnings when compiling with -Wstrict-prototypes (#157, #158).
• Removed wk_chunk_map_feature() in favour of using chunking strategies
  directly (#132, #159).
• Optimized wk_coords() for xy() objects (#138, #160).
• Added accessor methods for record-style vectors: rct_xmin(), rct_xmax(),
  rct_ymin(), rct_ymax(), rct_width(), rct_height(), crc_center(),
  crc_x(), crc_y(), crc_r(), xy_x(), xy_y(), xy_z(), and xy_m()
  (#144, #161).
• Added rectangle operators rct_intersects(), rct_contains(),
  and rct_intersection() (#161).