Remaining tasks for storage

[mohammadfawaz]

#646 introduces storage accesses and reassignment using the storage keywords. There are still a few things to do to mark the feature complete. Summarizing them here:

• Investigate the new trait-based approach for storage. #1399
• Validate impurity and that no storage opcodes are generated in a pure context #1151
• Warn on unnecessary storage annotations #1183
• Handle arrays in storage declarations #1182
  • Currently, something like storage.s.t.x works well if s and t are structs. However, storage.s.t.0 won't if t is a tuple. This might get better with the new parser.
  • The workaround is to do (storage.s.t).0 but that only works when reading. When writing, the whole tuple needs to be written.
  • Directly writing a tuple element of a tuple struct member doesn't work #905 is related to the above for non-storage accesses.
• Make sure the storage keyword is not valid in scripts. #1184
• Forbid heap types in storage: Forbid heap types in storage #1131
• Handle storage initializers #1185