…tr index

… the same for structs)

- introduce pytypes.TupleLikeType that captures both native and ARC4Tuples
- make a NamedTupleType subclass of TupleType that exposes a `fields` property which is easier to create & consume
- use NamedTuple for pre-compiled data structures
- correct implicit arc4 conversion to use iterate_static, which also ensure single evaluation
- introduce pytype for NamedTupleBaseType - this is somewhat of a lie, since typing.NamedTuple is not actually part of MRO at runtime due to Python magic, but it's a good lie because it allows resolving expression and type builders for NamedTuple without any hackery
- fixing some error messages to start with lower case
- expose concrete types on WType and PyType classes, this simplifies some things and Sequence/Mapping was mostly used as shorthand
- add some negative tests for NamedTuple scenarios
- general refactoring of new NamedTuple feature