Fix type level operators

[savq]

• Add return_type field to anonymous functions.
• Fix type_clause and where_clause:
  • Parse (<: Type) outside type definitions as a unary_expression.
  • Don't use type_clause after where_clause. Closes #108
  • Don't use multiple where_clause; parse the rest as where_expression instead (in practice you never need more than one where).
• Update tests.
• Update known-failures added in the previous PR. #106

NOTE: This PR doesn't require updating queries.