New semantic analyzer: Refactor handling of is_inferred_def & co

[ilevkivskyi]

There are several things that hold information about whether a type of variable should be inferred or not:

• RefExpr.is_new_def
• RefExpr.is_inferred_def
• Var.is_ready
• Var.is_inferred
• TypeChecker.is_definition()

Such a variety of things is partially caused by the fact that special definitions like named tuples etc. used to have a Var() to which the lvalue was pointing. With the new analyzer we don't have this, so it would be good to clean-up/refactor the logic around inference after we merge the two analyzers.

See also #6453