WL#5082: Value register implementation

As described in WL#4904, the goal is to introduce a Value object that
can represent a value of any type used in SQL expressions.  WL#4760
designed and prototyped a version based on immutable value objects.
As explained in WL#4904, this approach had some issues that lead us to
believe that a design based on "Value registers" are more appropriate.

Instead of passing around immutable values, we will pass around a
reference to a storage location for a piece of data, i.e. similar to
the concept of a computer register.  For example, Item could in order
to evaluate an expression provide a method "Item::eval(Register*)",
and store the result of the evaluation in the register provided by the
caller.

The Value register will provide a centralized API to access and
convert a single value (data) of SQL expressions.  That is, a Value
register may store a value of any SQL type.  Associated with the
register will be a metadata object that provide the necessary type
information for the value. (Ref. WL#5081).

Based on the requirements given in WL#4904, a Value register should
provide: 

1. Construction of a value for a parser constant - avoiding extra
   copying, if possible.  E.g., "SELECT "long text";  
   (Req 1 in HLS of WL#4904)

2. A central location for type data conversion functionality. This
   functionality should be accessed through an API. (Req 6)

3. Conversion of a Value to int, char, string, etc.  (Req 8)