Redefine struct getters to return their struct type, not a tuple of individual fields

With the AbiEncoderv2, is the intent to have structs returned from auto-generated getters for public storage?

For example,

```
struct Data {
uint256 x;
bytes32 y;
}

mapping (bytes32 => Data) public data;
```

This exposes the getter `function data(bytes32) public view returns (uint256,bytes32)`

Is it possible that it could instead expose `function data(bytes32) public view returns (Data)`?

Start with the AbiEncoderv2 behavior and the generated getter example in the issue. No files or tests are named, so trace the public mapping getter and related ABI encoder tests first; done requires a decided, tested getter return shape and documented compatibility behavior.

Written by the indexing model from the issue text.