raw_istream and kin: a few cleanups.

anchor() doesn't seem to change compiler output in current compilers, so
remove it.  Use default where appropriate rather than empty bodies.

src/raw_istream.cpp

@@ -11,8 +11,6 @@
 
 using namespace nt;
 
-void raw_istream::anchor() {}
-
 bool raw_mem_istream::read(void* data, std::size_t len) {
   if (len > m_left) return false;
   std::memcpy(data, m_cur, len);