Add containers.agda-lib to lib directory

[HeinrichApfelmus]

DO NOT MERGE! Please merge #405 first.

This pull request adds a library lib/containers/containers.agda-lib which imports large chunks of the Data.Map and Data.Set modules from the containers package, including proofs.

Comments

• I have chosen to not add the prefix Haskell. to the module names Data.Map.Prop and Data.Set.Prop, because:

  • Strictly speaking, these modules do not correspond to an existing Haskell module, …
  • … but the properties proven have value as exported Haddock comments. In other words, once we can export properties to comments (related to Preserve comments #114), we can later make Data.Map.Prop part of, say, a containers-prop.cabal package and make the proven properties available as a Haskell package. (The properties cannot be reused for proofs in Haskell Land, but at least they can be exported as "this has been proven!" to Haskell Land.)

• I was not quite sure whether to choose the name Data.Set.Prop or Data.Set.Law. John Hughes appears to use both "law" and "property" synonymously, while using "law" more often. Perhaps a good distinction is that the "laws" are closer to being complete while "properties" are mostly "haphazard" consequences.