[IsaacFleetwood]

Update the typing implementation to be consistent with the new typing rules outlined in the document.
Specifically:

• Nothing no longer exists as a value type, it has been separated out into a separate properties (props) field.
• The rules have been updated to propagate properties as stated in the typing rules.
• Parameters have been rewritten to read better, and objects have been redefined to be an explicitly recursive definition instead of an array.

Still To-Do:

• Determine consistent name for a value type versus a type tuple (tuple of a value type and its properties).
  Right now, the code is a bit all over the place for these names, given these once were the same thing.
• Calculate type of filter variables by a pass over the entire AST before type checking. The type should be the fixpoint of the intersection of usages, as defined in the typing document.
• Determine way of passing Non-Empty type guarantees to the compiler. This is necessary to give compile-time guarantees that object accesses will not return nothing. Importantly, this should include non-empty guarantee of not just keys but also intersection types.

Requested review from Ran given the work being done in the SQL-New codegen. Given the changes, it's likely this will require consolidation with other branches being worked on to fix certain breaking changes.

[TiarkRompf]

I'll let @guo543 comment as well, but I'd be otherwise ready to merge this.

Some purely cosmetic points:

• let's stick to some uniform coding style regarding whitespace and use if (...), for (...) instead of if(...), for(...))
• "nothing" is a better name for the empty type than "never"

[guo543]

Looks good! Thanks for also changing the sql codegen.