memoise 2.0.0

• Memoise now uses caching objects from the cachem package by default. These caches support automatic pruning, so that they won't grow indefinitely. The older-style cache objects in the memoise package are still supported, but we suggest using new-style caches from cachem. (#112)
• Name clashes between function arguments and variables defined when memoising
  no longer occur (@egnha, #43).
• Add Google Cloud Storage support via cache_gcs() (@MarkEdmondson1234, #59)
• Add compress option for non-memory caches (@coolbutuseless, #71).
• Use absolute path in cache file system backend, so user can change working
  directory after using relative path (@xhdong-umd, #51, #65)
• Add drop_cache() to drop the cached result for particular arguments
  (@richardkunze, #78)
• Suppress messages of aws.s3::head_object within cache_s3's cache_has_key
  to avoid printing of 404 messages for new keys (@stelsemeyer, #96).