<sstream>: Change the underlying buffer of basic_stringbuf to basic_string

See: https://github.com/microsoft/STL/pull/919#issuecomment-650688421

The underlying buffer of basic_stringbuf is simply an array, which makes implementing P0408R7 hard.

When there's an rvalue of basic_string received, we cannot simply replace the current buffer with the string buffer:

• basic_string uses SSO (small string optimization). When it is in small mode, the string should be copied to the current buffer;
• In readonly mode, basic_stringbuf doesn't recognize the reserved space of the string, which means we should copy the string non-conditionally.

When we want the basic_string from an rvalue of basic_stringbuf, we cannot simply construct a basic_string from the current buffer:

• The current buffer is not zero-terminated. When the buffer is already full, we should copy the buffer to a basic_string;
• It loses SSO, because basic_stringbuf doesn't uses SSO.

We could see that the current implementation will cause a lot of copying. I think it should be fixed in vNext.

vNext note: Resolving this issue will require breaking binary compatibility. We won't be able to accept pull requests for this issue until the vNext branch is available. See #169 for more information.

Start with the basic_stringbuf implementation and the linked P0408R7 discussion. Review how rvalue basic_string and basic_stringbuf cases interact with SSO, termination, reserved space, and readonly mode. A contribution cannot be accepted until the vNext branch is available, and completion would require a design that avoids the described copying while preserving compatibility requirements.