feat(decompile): support nested mappings

### Component

Heimdall (Core)

### Describe the feature you would like

heimdall only supports 1d mappings, i.e. 

```solidity
mapping(bytes32 => bytes32) public stor_map_a;
```

however, we want to support nested mappings:

```solidity
mapping(bytes32 => mapping(bytes32 => bytes32)) public stor_map_b;
```

This should be enabled by #242 