Make StringPiece match std::string_view

**Is your feature request related to a problem? (你需要的功能是否与某个问题有关?)**

C++17 will include `std::string_view`, which is essentially just a standardized replacement for StringPiece. To plan ahead for this, we should consider updating the StringPiece API so that it exactly matches std::string_view. Then a few years from now when C++17 is widespread we can make StringPiece just a type alias for std::string_view. I believe StringPiece is just used within protobuf internals at the moment and so we can safely change its API without breaking any user code.

**Describe the solution you'd like (描述你期望的解决方法)**

Using a macro to using string_view

```
#ifdef XXX
using base = basic_string_view
...
#else
// The original definitions..
#endif
```

The abseil and boost solve it in this way.

**Describe alternatives you've considered (描述你想到的折衷方案)**

Using constructor like `StringPiece(string_view)`...

**Additional context/screenshots (更多上下文/截图)**

None.

No file or test is named. Start by locating the StringPiece implementation and its internal usages, then compare its API with C++17 std::string_view and review the proposed macro-based compatibility approach. Done means the supported StringPiece API matches std::string_view closely enough for a future type alias without breaking current internal use.

Written by the indexing model from the issue text.