Add function expr parsing

* Introduce new `fn` keyword
* Function example is added to examples/expr.not

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

src/syn/parser.y

@@ -92,10 +92,25 @@ IndexExpr -> Result<Expr>:
     ;
 
 AccessExpr -> Result<Expr>:
-        AtomExpr { $1 }
+        FunExpr { $1 }
     |   CallIndexExpr '.' Ident { Ok(AccessExpr::new_expr($1?, $3?)) }
     ;
 
+FunExpr -> Result<Expr>:
+        AtomExpr { $1 }
+    |   'fn' '(' FunParams ')' '{' Body '}' { Ok(FunExpr::new_expr($3?, $6?)) }
+    ;
+
+FunParams -> Result<Vec<String>>:
+        FunParamsTail { $1 }
+    |   { Ok(Vec::new()) }
+    ;
+
+FunParamsTail -> Result<Vec<String>>:
+        FunParamsTail ',' Ident { flatten($1, $3) }
+    |   Ident { Ok(vec![$1?]) }
+    ;
+
 AtomExpr -> Result<Expr>:
         Atom { $1.map(Expr::Atom) }
     |   '(' Expr ')' { $2 }