extend chalk `TypeName` with builtin types

[nikomatsakis]

The TypeName enum needs to be extended to cover the "built-in" types from Rust. The following variants would eventually want to be added:

• scalar types:
  • unsigned integer types (u32, u64, usize, etc)
  • signed integer types
  • bool
  • char
• tuple types (parameterized by arity, so Tuple(usize))
• str -- see extend TypeName with str type #437
• reference types & and &mut -- create TypeName for & and &mut #438
• raw pointer types * const and *mut -- create typename for raw pointers #439
• array types (requires const work to land)
• slice type -- extend TypeName with slice type #440
• FnDef (PR: Add FnDef to TypeName #449)
• Never
• Closure
• generator/ generator witness -- Implement generators #593, create a "generator witness" type #504
• generalize the builtin struct type to adt to match rustc #453: generalize struct to enum/union
• Support for multiple variants on Adt -- extend chalk ADTs with information about variants #505
• Extern types -- Support extern opaque types. #595

(Need to update this list, not yet complete, see this section of the chalk book for a more complete list as well.

[MarkMcCaskey]

I'm interested in taking this on! I'll follow up with a PR

[MarkMcCaskey]

@rustbot claim

[basil-cow]

@MarkMcCaskey what's the status on this?

[MarkMcCaskey]

Sorry about the radio silence -- I've been busier than I anticipated recently and haven't had the spare focus to work on this. I haven't worked on closures or functions other than what was in the original PR yet. I can dedicate a bit of time to it this weekend and more time to it next week. If this is blocking other issues and/or if someone wants to implement it, I have no problem with someone else taking it on but I'd be happy to work on it otherwise!