Enforce Syntax Trees Invariantrs

[matklad]

We have couple of weak invariants about our syntax trees, which are not really enforced and sometimes are broken in practice. We should

• document invariants
• assert them
• make sure that they are obeyed in practice

Here's the tentative list of invariants:

• The only node that is allowed to be empty is SourceFile
• The only token kind that is allowed to be empty are fake dollar parens from macro expansion
• Braces should play well with incremental reparsing (see Improve error recovery for unclosed use tree lists. #4680 (comment))

Notably, the following is not an invariant, but we rely on it in some ast extensions:

• Parent nodes are not guaranteed to exist.

For example, ast::EnumVariant should return Option

[matklad]

#4680 is something we neeed to look at as well.