Adding update and extend type support for data types

[mkolosick]

Allows extend and update syntax to be used on data types.
Errors if the fields do not occur in the data type, e.g.

data Foo:
  | bar(a :: Number)
end

bar(1).{b: 3}

[blerner]

Are these test cases named correctly? This doesn't seem to be a missing field, it's a wrong type. In general, could you add a comment to each bad test, above the failing expression, explaining what the error should be?

[mkolosick]

Oops I flipped two of the test files for both update and extend.

[blerner]

This looks almost right...but you forgot to fix this specific file. After that, I'm good to merge this.

[mkolosick]

I have fixed it now.

[blerner]

Thanks -- just to confirm: it is a design goal of the type checker to disallow adding fields to data values, right?

If so, I'm good to merge...

[mkolosick]

That is correct.

[jpolitz]

Wow, we got static support for this feature before dynamic support is fully baked.

Currently, update of existing fields does weird things dynamically, as does adding new fields.

#835 is currently an error.

See:

https://github.com/orgs/brownplt/teams/pyret/discussions/8

I think we need a decision on the desired dynamic semantics of this before we can start blessing programs as typed.