Lift restrictions against allocating the last storage slot

Currently, defining a storage variable that covers the last storage slot results in a compilation error. Either when contract's variables are so large that they cover the whole storage (this is already possible) or when the layout base is shifted so much that reasonably-sized variables reach the last slot (which will be possible after #597).

It's not clear why the restriction was added. It may have been done to keep the last slot reserved, but this wasn't documented anywhere, so it could just as well have been be a mistake.

We should remove the restriction.

### Steps to reproduce
#### Oversized array
```solidity
contract C {
uint[2**256] x;
}
```
```
Error: Array length too large, maximum is 2**256 - 1.
--> test.sol:2:10:
|
2 | uint[2**256] x;
| ^^^^^^
```

#### Layout
```solidity
contract D layout at 2**256 - 1 {
uint x;
}
```
```
Error: Contract extends past the end of storage when this base slot value is specified.
--> test.sol:1:22:
|
1 | contract D layout at 2**256 - 1 {
| ^^^^^^^^^^
```

No source file or test is named. Start by tracing the two reported compiler diagnostics for the oversized array and the layout base at 2**256 - 1, then verify that both examples compile without the last-slot restriction and that storage bounds remain correct.

Written by the indexing model from the issue text.