poly: add poly.constant folding and materialization

From https://github.com/google/heir/pull/94#issuecomment-1679029556

> we should add a poly.constant op to directly convert a #poly.polynomial to a !poly.poly without the need to go via tensors

The idea is to have an op that is analogous to `arith.constant`, which, by the use of an array attribute, can populate a tensor of constants, not just a single value, as in

```
%coeffs1 = "arith.constant"() <{value = dense<[2, 2, 5]> : tensor<3xi32>}> : () -> tensor<3xi32>
```

We could have something similar like

```
#cycl_2048 = #poly.polynomial<1 + x**1024>
#ring = #poly.ring
%poly1 = "poly.constant"() <{coefficients = dense<[2, 2, 5]> : tensor<3xi32>}> : !poly.poly<#ring> : () -> !poly.poly<#ring>
```

The attributes are already implemented, and so we would just need the op and lowerings.