Data locations should be tied to types, not variable declarations.

## Abstract

Solidity could allow to have `storage` references in `memory` structures or even `memory` arrays.

## Motivation

Sometimes it is handy to have memory structure with some storage refs. Now we can use this trick, uncomfortable but works:
```solidity
type DataPtr is uint256;

function get(DataPtr ptr) internal pure returns(Data storage data) {
assembly {
data.slot := ptr
}
}
```

## Specification

It would be convenient to have `storage` refs in `memory` struct by allowing following syntax:

```solidity
struct Data {
// ...
}

struct Object {
Data storage data; // <- allow storage keyword here
bytes32 context;
}
```

No repository file, test, or entry point is named. Start by reviewing the compiler's existing type-system and data-location handling, then trace how struct member declarations are parsed and validated. Done would require a decided design for storage references in memory structures or arrays, support for the specified syntax, and tests covering the resulting semantics.

Written by the indexing model from the issue text.