DRAFT: Better handle ambiguous parsing

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This is A DRAFT. Feedback is welcome, but understand this will take a number of days to complete. I will post my work at the end of every day that I work on it. Hit me on Zulip if you want to talk about the approach in more detail.

Here is the high level plan:

• Untyped parse nodes
• Unified structure for all nodes that appear in two or more node types (Pattern, Expr, and TypeAnno)
• Add parseUntypedNode function and related bits to parser
  • Only called in ambiguous parsing contexts like: Beginning of statement that does not state with keyword; Parsing top of block/record in expr parsing.
• Untyped scratch
• Functions to convert from untyped to typed nodes (hopefully just change tag)
• After conversion, continue parsing where it would have been if we knew the type from the start.