Delicately re-layer Operation, Statement, and OperationStmt, reworking

#includes so Statements.h includes Operation.h but nothing else does.  This is
in preparation to eliminate the Operation class and the complexity it brings
with it.  I split this patch off because it is just moving stuff around, the
next patch will be more complex.

This is step 14/n towards merging instructions and statements, NFC.

PiperOrigin-RevId: 227071777