Combine the Expr.pTab and Expr.pWin fields into a union named "y". Add a new

Combine the Expr.pTab and Expr.pWin fields into a union named "y".  Add a new
EP_WinFunc property that is only true if Expr.y.pWin is a valid pointer.
This reduces the size of the Expr object by 8 bytes, reduces the overall
amount of code, and shaves over 1 million cycles off of the speed test.

FossilOrigin-Name: ad130bb86e74e6ce165fdbdce3a19699510f0e62071c1c7923b5a4538d888c7c