Support for decimals up to 38 digits

Currently DECIMAL only supports 30 decimals.
We should extend support to at least 38 decimals, to be more compatible with financial applications that depends on this limit

FLOAT and DOUBLE should still have the old limit of 30 decimals.

The change should be done so there is a minimum of notable change for old clients, except if they are using decimal fields with more than 30 decimals.

One change that is unavoidable is that decimal expressions will return results with more than 30 decimal and that if one converts a string to decimal it will have up to 38 decimals.

CREATE ... SELECT with a decimal with > 30 decimals will create a column
with a smaller range than before as we are trying to preserve the number of
decimals.

CREATE TABLE t1 SELECT 123456789012345678901234567890123456789012345.123456789012345678901234567890123456789012345;
Will change the value from:
99999999999999999999999999999999999.999999999999999999999999999999
to
99999999999999999999999999999.999999999999999999999999999999999999