`keccak256()` in comptime context

## Abstract

Implement evaluation of `keccak256()` in comptime context.

While this is technically implementable without prerequisites, it won't be usable in practice without implementing at least stage 1 of #16420.

```solidity
contract C layout at uint(keccak256("my.contract.id")) {}
```

## Motivation
The goal is to allow the use of `keccak256()` builtin in `layout at` specifiers.

There are two layout base expressions in common use: [`erc7201`](https://eips.ethereum.org/EIPS/eip-7201) and plain hashing of a string using `keccak256()`. We're adding a builtin for the former in #15968, but for completeness we should also support the latter.

The main hurdle here is the return type. `keccak256()` returns `bytes32` while the layout base expressions must be integers. We also do not support type conversions in compile-time context so constant expressions of that type cannot be simply converted.

## Backwards Compatibility

Fully backwards-compatible.