Fix precedence for the NOT operator (2/4)

I checked the docs of a few of the most popular RDBMSes, and it seems
there's consensus that the precedence of `NOT` is higher than `AND`,
but lower than `IS NULL`.

Postgresql[1], Oracle[2] and MySQL[3] docs say that explicitly.

T-SQL docs[4] do mention it's higher than `AND`, and while they don't
explicitly mention IS NULL, this snippet:

    select * from (select 1 as a)x
    where (not x.a) is null

...is a parsing error, while the following works like IS NOT NULL:

    select * from (select 1 as a)x
    where not x.a is null

sqlite doesn't seem to mention `NOT` precedence, but I assume it works
similarly.

[1] https://www.postgresql.org/docs/current/sql-syntax-lexical.html#SQL-SYNTAX-OPERATORS
[2] https://docs.oracle.com/cd/B19306_01/server.102/b14200/conditions001.htm#i1034834
[3] https://dev.mysql.com/doc/refman/8.0/en/operator-precedence.html
[4] https://docs.microsoft.com/en-us/sql/t-sql/language-elements/operator-precedence-transact-sql?view=sql-server-2017

tests/sqlparser_generic.rs

@@ -182,6 +182,30 @@ fn parse_is_not_null() {
     );
 }
 
+#[test]
+fn parse_not_precedence() {
+    use self::ASTNode::*;
+    // NOT has higher precedence than OR/AND, so the following must parse as (NOT true) OR true
+    let sql = "NOT true OR true";
+    match verified_expr(sql) {
+        SQLBinaryExpr {
+            op: SQLOperator::Or,
+            ..
+        } => assert!(true),
+        _ => assert!(false),
+    };
+
+    // But NOT has lower precedence than comparison operators, so the following parses as NOT (a IS NULL)
+    let sql = "NOT a IS NULL";
+    match verified_expr(sql) {
+        SQLUnary {
+            operator: SQLOperator::Not,
+            ..
+        } => assert!(true),
+        _ => assert!(false),
+    };
+}
+
 #[test]
 fn parse_like() {
     let sql = String::from("SELECT * FROM customers WHERE name LIKE '%a'");