Add support for record specs

[brettbeatty]

Records are a specialized tuple, and they have a typespec format that closely resembles that of tuples. They are not used frequently in Elixir, where structs are preferred, but they still come up. Notably, MapSet.t/1 builds upon :sets.set/1, which is a union of an old record-based type and a new map-based type. Even though MapSet always uses the map-based implementation under the hood, the typespec includes record typespecs. This is the underlying cause of #146 and #147.

Support for records requires two minor changes:

• Match on records in TypeEngine. The type structure for records is the same as that of tuples, so we can just copy the tuple clauses and repurpose them for records.
• Handle named elements in TypeMatchError. The existing type_to_string/1 assumes there will be at most one name in the spec and that it will be at the top level (for named args). Record typespecs, however, provide a name for each field. This PR preserves the existing format for named args but also allows named components within specs.