Support named arguments in function invocations (#250)

This commit supports functions with argument names.

the format is :
"Select some_function( a => exp, b => exp2 .. ) FROM table1
OR
"select * from table(function(a => exp)) f;"

see:
https://jakewheat.github.io/sql-overview/sql-2016-foundation-grammar.html#named-argument-assignment-token
or the motivating example from snowflake:
https://docs.snowflake.com/en/sql-reference/functions/flatten.html

src/tokenizer.rs

@@ -99,6 +99,8 @@ pub enum Token {
     LBrace,
     /// Right brace `}`
     RBrace,
+    /// Right Arrow `=>`
+    RArrow,
 }
 
 impl fmt::Display for Token {
@@ -139,6 +141,7 @@ impl fmt::Display for Token {
             Token::Pipe => f.write_str("|"),
             Token::LBrace => f.write_str("{"),
             Token::RBrace => f.write_str("}"),
+            Token::RArrow => f.write_str("=>"),
         }
     }
 }
@@ -400,7 +403,13 @@ impl<'a> Tokenizer<'a> {
                         _ => Ok(Some(Token::Pipe)),
                     }
                 }
-                '=' => self.consume_and_return(chars, Token::Eq),
+                '=' => {
+                    chars.next(); // consume
+                    match chars.peek() {
+                        Some('>') => self.consume_and_return(chars, Token::RArrow),
+                        _ => Ok(Some(Token::Eq)),
+                    }
+                }
                 '.' => self.consume_and_return(chars, Token::Period),
                 '!' => {
                     chars.next(); // consume
@@ -766,6 +775,23 @@ mod tests {
         compare(expected, tokens);
     }
 
+    #[test]
+    fn tokenize_right_arrow() {
+        let sql = String::from("FUNCTION(key=>value)");
+        let dialect = GenericDialect {};
+        let mut tokenizer = Tokenizer::new(&dialect, &sql);
+        let tokens = tokenizer.tokenize().unwrap();
+        let expected = vec![
+            Token::make_word("FUNCTION", None),
+            Token::LParen,
+            Token::make_word("key", None),
+            Token::RArrow,
+            Token::make_word("value", None),
+            Token::RParen,
+        ];
+        compare(expected, tokens);
+    }
+
     #[test]
     fn tokenize_is_null() {
         let sql = String::from("a IS NULL");