LibCore+Everywhere: Make Core::Stream read_line() return StringView

Similar reasoning to making Core::Stream::read() return Bytes, except
that every user of read_line() creates a StringView from the result, so
let's just return one right away.

Userland/Libraries/LibWebSocket/Impl/WebSocketImpl.cpp

@@ -63,8 +63,8 @@ ErrorOr<ByteBuffer> WebSocketImpl::read(int max_size)
 ErrorOr<String> WebSocketImpl::read_line(size_t size)
 {
     auto buffer = TRY(ByteBuffer::create_uninitialized(size));
-    auto nread = TRY(m_socket->read_line(buffer));
-    return String::copy(buffer.span().slice(0, nread));
+    auto line = TRY(m_socket->read_line(buffer));
+    return line.to_string();
 }
 
 }