Rename paddle::strings::StringPiece into paddle::string::Piece

[wangkuiyi]

So we can write the following in paddle/framework/operator.h or somewhere in the paddle namespace:

namespace paddle {
namespace framework {
namespace detail {
namespace whatever {
...
string::Piece p = string::Printf("%2.2f", 12,345);
...

C++ compiler would be able to find out a parent namespace, which will be ::paddle, that has string::Piece.

For a verification please see https://gist.github.com/wangkuiyi/0bb245a71d3dcfc77e1e4d6961023033.

[helinwang]

LGTM!