common/StackStringStream: optimize xsputn() and overflow()

common/StackStringStream: optimize xsputn() and overflow()

use the internal buffer semantics provided by
basic_streambuf, in hope to improve the performance when a single
character is inserted into buf.

* xsputn(): use memcpy() directly instead of creating a temporary
  string_view
* overflow(): overflow() should be called only if the internal buffer
  overflows when a single character is put into the streambuf. so
  we can use the fact that the size of inserted buffer is known,
  and use push_back() instead.
  also, please see libstdc++'s implementation of basic_streambuf::sputc(),
  where __builtin_expect() is used to instruct the compiler that
  the non-overflow branch is more likely to be chosen. so, we need
  to optimize for that branch. hence this change.
* remove the no-more-used push() method

Signed-off-by: Kefu Chai <kchai@redhat.com>