add SQL LENGTH() function as alias for len()

[mccanne]

No description provided.

[mattnibs]

Put this in runtime/ztests/expr/function where the test for len is

[philrz]

Verified in super commit 5c1851b.

Before:

$ super -version
Version: v1.18.0-156-gb2c46ef7

$ super -c "SELECT LENGTH('W3Schools.com');"
no such function at line 1, column 8:
SELECT LENGTH('W3Schools.com');
       ~~~~~~~~~~~~~~~~~~~~~~~

Now:

$ super -version
Version: v1.18.0-157-g5c1851be

$ super -c "SELECT LENGTH('W3Schools.com');"
{length:13}

Thanks @mccanne!