Add syntax sugar for struct fields (#56)

src/antlr4/Navi.g4

@@ -95,11 +95,14 @@ args: (expression (',' expression)* ','?)?;
 fields: '{' (field (',' field)* ','?)? '}';
 fieldsSpread:
 	'{' (
-		spread (',' spread)* (',' (field (',' field)* ','?)?)?
-		| (field (',' field)* ','?)?
+		spread (',' spread)* (
+			',' (fieldMaybeAssert (',' fieldMaybeAssert)* ','?)?
+		)?
+		| (fieldMaybeAssert (',' fieldMaybeAssert)* ','?)?
 	) '}';
 spread: OpSpread expression;
 field: Identifier assert;
+fieldMaybeAssert: Identifier assert?;
 parameters: '(' (parameter (',' parameter)* ','?)? ')';
 varArgParameters:
 	'(' (

src/antlr4/NaviListener.js

@@ -302,6 +302,15 @@ export default class NaviListener extends antlr4.tree.ParseTreeListener {
 	}
 
 
+	// Enter a parse tree produced by NaviParser#fieldMaybeAssert.
+	enterFieldMaybeAssert(ctx) {
+	}
+
+	// Exit a parse tree produced by NaviParser#fieldMaybeAssert.
+	exitFieldMaybeAssert(ctx) {
+	}
+
+
 	// Enter a parse tree produced by NaviParser#parameters.
 	enterParameters(ctx) {
 	}