i.e. ASC/DESC/unspecified - so that we don't lose information about
source code.
Also don't take any keyword other than ASC/DESC or Comma to mean
'ascending'.
...as this syntax is not specific to the PostgreSQL dialect.
Also use verified() to assert that parsing + serializing results in the
original SQL string.
Mainly by replacing `assert_eq!(sql, ast.to_string())` with a call to
the recently introduced `verified()` helper or using `parses_to()` where
the expected serialization differs from the original SQL string.
There was one case (parse_implicit_join), where the inputs were different:
let sql = "SELECT * FROM t1,t2";
//vs
let sql = "SELECT * FROM t1, t2";
and since we don't test the whitespace handling in other tests, I just
used the canonical representation as input.
