New type: Integer

I've added a new pseudotype, "Integer".  Please use it instead of Number
when you really expect or produce integers.  If it's a function's input, please
identify the function conversion operation (often INT), and be SURE to write
tests that verify it.

Obviously, this isn't really "new".  I'd avoided creating an Integer type because
NOT all functions that expect integers use the same conversion operations
(e.g., the text string operations tend to use INT, but ISEVEN/ISODD use TRUNC).
But I've realized that having "Integer" in the type signature is still very
valuable information, and if we don't mandate a particular integer conversion
operation, it serves as a valuable hint for the need to specify the conversion
(or, alternatively, to state that it's implementation-defined).

I've written some draft text, and converted a few text operations to use
Integer, to show what it looks like.  The next draft (about to be posted)
shows that.

--- David A. Wheeler