WL#966: Implement CHECKSUM and CHECKSUM_AGG functions

We should implement the following checksum functions (row and aggregate) for use
in SQL queries, and emulate SQL Server syntax.

Microsoft SQL Syntax:
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CHECKSUM
BINARY_CHECKSUM
Returns the checksum value computed over a row of a table, or over a list of
expressions. CHECKSUM is intended for use in building hash indices.

Syntax
CHECKSUM ( * | expression [ ,...n ] ) 
BINARY_CHECKSUM ( * | expression [ ,...n ] ) 

Arguments
*

Specifies that computation is over all the columns of the table. This 
should work with all types and any combinations of types.

The difference between BINARY_CHECKSUM an CHECKSUM is that
CHECKSUM(a)=CHECKSUM(b) if a=b
(or both are NULLs) - so for strings it's charset and collation issue.
As manual says - CHECKSUM is intented for building hash indexes.

BINARY_CHECKSUM's are equal is values are identical byte-wise.It is useful for
tracking changes.

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CHECKSUM_AGG
Returns the checksum of the values in a group.

Syntax
CHECKSUM_AGG ( [ ALL | DISTINCT ] * | expression [,...])

Arguments
ALL

Applies the aggregate function to all values. ALL is the default.

DISTINCT

Specifies that CHECKSUM_AGG return the checksum of unique values.

expression

Is a constant, column, or function, and any combination of arithmetic, 
bitwise and string operators. Aggregate functions and subqueries are not 
allowed.

This function is aggregate version of CHECKSUM. As aggreate binary checksum
Microsoft suggests CHECKSUM_AGG(BINARY_CHECKSUM(...))

MySQL Remarks
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I don't really see the need of charset-aware CHECKSUM. Probably we'll just do it
binary. Also BINARY_CHECKSUM() can be added as an alias for CHECKSUM() if necessary.

The order of the rows in the table does not affect the result of 
CHECKSUM_AGG.