Check that tokens are non-empty after #Layout

[Scott-Guest]

Related to #4592

Check that, after removing all tokens matching #Layout, every token is still a non-empty regular expression, preventing bugs like

syntax #Layout ::= r"([\\n \\ \\r\\t])"
syntax Stmt ::= List{Int, " "}

where the list separator will never actually be matched because #Layout strips all white-space.

Explicitly, let L be the #Layout regex and S a token. We must check that (~(*L*))&S is non-empty where ~ is negation, * is Kleene star, and & is intersection.