EDSC: move Expr and Stmt construction operators to a namespace

In the current state, edsc::Expr and edsc::Stmt overload operators to construct
other Exprs and Stmts.  This includes some unconventional overloads of the
`operator==` to create a comparison expression and of the `operator!` to create
a negation expression.  This situation could lead to unpleasant surprises where
the code does not behave like expected.  Make all Expr and Stmt construction
operators free functions and move them to the `edsc::op` namespace.  Callers
willing to use these operators must explicitly include them with the `using`
declaration.  This can be done in some local scope.

Additionally, we currently emit signed comparisons for order-comparison
operators.  With namespaces, we can later introduce two sets of operators in
different namespace, e.g. `edsc::op::sign` and `edsc::op::unsign` to clearly
state which kind of comparison is implied.

PiperOrigin-RevId: 233578674