Add missing <algorithm>/<iterator> includes in Serialization.hpp and Results.hpp

[jonathon-bell]

These two headers use standard library facilities without including the headers that declare them — they always relied on some other header pulling in <algorithm>/<iterator> transitively. That's incidental, not guaranteed by the standard, and recent libc++ releases (Clang 23) no longer do it, so the build now fails:

$ cmake -S . -B build && cmake --build build
...
include/rapidcheck/detail/Results.hpp:26:22: error: no member named 'back_inserter' in namespace 'std'
      iit, end, std::back_inserter(out.shrinkPath));
                     ^~~~~~~~~~~~~
include/rapidcheck/detail/Serialization.hpp:48:10: error: no member named 'copy' in namespace 'std'
  return std::copy(begin(value), end(value), oit);
         ^~~~~~~~~

• Serialization.hpp uses std::copy and std::distance but includes neither <algorithm> nor <iterator>.
• Results.hpp uses std::back_inserter but does not include <iterator>.

Fix: add the missing includes directly to the headers that use them (IWYU). Reproduces the same way under -std=gnu++11 (rapidcheck's own project default) as under later standards — this isn't a C++17/20/23 compatibility issue, just a newer standard library shuffling its internal include graph. Same category as #345 (missing <exception> in Gen.hpp).

This should hold regardless of whether it reproduces on any particular CI toolchain: the standard never specifies that <unordered_map>/<string>/etc. must provide <algorithm> or <iterator>, so code relying on that is fragile by construction even where it currently happens to build. The fix is a pure addition of includes — zero behavioral change, and it cannot break any configuration that builds today.

Environment used to reproduce and verify: macOS, Clang 23.1.0, libc++, CMake, default -std=gnu++11 (rapidcheck's own build default — verified via cmake --build, independent of any consuming project's language standard).

Verified cmake --build succeeds for the rapidcheck target after the fix.