extend `TypeName` with `str` type

[nikomatsakis]

As part of #368, we need to extend chalk's TypeName to model the built-in str type.

You can follow #394 which provides a rough model of what needs to be extended. str should be roughly like u32.

You would need to

• add str to TypeName
• extend parser to accept str as a type name, and add a suitable node to the AST
• Update the current state table to include str. I don't think it requires any special handling, its impls live in the standard library (and it does not implement Sized).

This issue has been assigned to @Mcat12 via this comment.

[nikomatsakis]

cc @Mcat12 -- maybe you are interested in claiming this issue?

[AzureMarker]

@rustbot claim