scan+parse+datum: remove r5js.Datum#unescapeStringLiteral.
datum: move r5js.Datum#getMacro to r5js.ast.Macro.
datum: put r5js.ast.Identifier in its own file.
datum+runtime: inline the one use of r5js.Datum#quote.
ast+misc: rename r5js.Quote => r5js.ast.Quote.
ast: move r5js.ast.Quote into its own file.
macro: rewrite addTemplateBinding() to use r5js.Datum#forEachChild.