Add last-expression-return-value to functions as well

We did it for if and block statements, now functions too support the
last expression being its return value. If there isn't an expression
(e.g. an assign statement) we just return `nil`.

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

tools/genast.py

@@ -243,7 +243,7 @@ GENERATE = [
             "lparen: Token",
             "params: Vec<(Token, Option<ExprP>)>",
             "return_type: Option<ExprP>",
-            "body: Vec<StmtP>",
+            "body: BlockExpr",
             "rbrace: Token",
         ],
     )