Currently, variant keys can either be numbers (NumberExpressions) or text (VariantNames). Text keys allow inline whitespace; the whitespace at the extremes, however, is trimmed. Special characters like { or [ are forbidden, but no escape sequences are allowed either.

This PR changes variant keys to now be numbers (as before), identifiers (Identifier) or quoted text (StringExpressions). The VariantName AST node has been removed.

Fixes #90.