Add map literals

Map literals share the brackets with lists, but they use colons to
delimit keys and values.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

tools/genast.py

@@ -229,7 +229,10 @@ GENERATE = [
             "rbrace: Token",
         ],
     )
-    .add_struct("List", ["lbracket: Token", "exprs: Vec<ExprP>", "rbracket: Token"]),
+    .add_struct("List", ["lbracket: Token", "exprs: Vec<ExprP>", "rbracket: Token"])
+    .add_struct(
+        "Map", ["lbracket: Token", "pairs: Vec<(ExprP, ExprP)>", "rbracket: Token"]
+    ),
     # Stmt
     GenerateGroup("Stmt")
     .add_struct("Import", ["import_kw: Token", "what: Vec<Token>", "module: Token"])