Wrap var"" nonstandard identifiers in var nodes

[c42f]

Useful to hold associated trivia (delimiter and var prefix) and indicate in a clean way that var"" was used.

Part of #88

[c42f]

Hmm ok. So k == K"Identifier" doesn't work for identifiying identifiers after this change. Not hard to fix, just need to decide between

• Using K"var" for the head and adding an is_identifier predicate
• Using K"Identifier" for the head

Also the presence of var"" needs to be covered by the parser tests in various places after this change

[c42f]

So... I feel like

• A K"var" head is a nice unambiguous representation of the concrete syntax
• A plain symbol (as exists currently and as Expr uses) is clearly a nice representation of the abstract syntax
• An internal node with head K"Identifier" would be a weird halfway solution between these

So let's go with K"var" head I guess