Controversial comparison behaviour of decimal literals with scale >30

Two decimal literals:
0.1111111111222222222233333333334444444444
and
0.111111111122222222223333333333
are considered as non-equal by "=",
but considered as equal for UNION DISTINCT:

MariaDB [test]> SELECT 0.1111111111222222222233333333334444444444 = 0.111111111122222222223333333333 AS cmp;

+-----+

| cmp |

+-----+

|   0 |

+-----+

1 row in set (0.00 sec)

MariaDB [test]> SELECT 0.1111111111222222222233333333334444444444 AS a UNION SELECT 0.111111111122222222223333333333;

+----------------------------------+

| a                                |

+----------------------------------+

| 0.111111111122222222223333333333 |

+----------------------------------+

1 row in set (0.01 sec)

Comparison in "=" and comparison in UNION DISTINCT should use the same rules.

The problem most likely happens because we don't support fields with decimal
scale > 30. UNION uses a temporary table and creates a field,
while '=' does not need a field and compares the two numbers directly.

Literals should probably be fixed not to support more than 30 fractional as well.