Feature Request: Get object's raw source

Hi,

How are you? Thanks of all, thanks for doing the port of the tool, I was thinking of doing it myself and I found someone else did it :smile:

I was wondering if there was a way to get the raw source of an objects, let's say a contract or function.

I was thinking in something like:

```
sourceUnitObject = parser.objectify(sourceUnit)
raw_text = sourceUnitObject.contracts['MyContract'].raw
print(raw_text)
>>'contract MyContract { ..'
```

Thanks!

先追蹤 parser.objectify(sourceUnit) 和 parser 的 ANTLR4 文法，以了解 contract 物件和 function 物件是如何建立的。確定如何為這些物件保留來源文字，然後驗證 sourceUnitObject.contracts['MyContract'].raw 是否回傳預期的 contract 文字，以及等效的 function 存取是否運作。

由索引模型根據 Issue 內容生成。