Automated fix by @leftibot.

What changed

Fix #612: Allow fun(&overloaded) for overload disambiguation
The cheatsheet's "preferred" syntax for binding overloaded free functions,
fun<Ret(Args...)>(&overloaded), only worked on MSVC because the existing
fun(T&&) overload sets T to a function type when called explicitly,
which produces a function rvalue-reference parameter that does not match
a function pointer argument. Standards-conforming compilers (clang, GCC
in some modes) correctly rejected it.
Added two new fun overloads, parameterized on the explicitly-supplied
function-type Signature and using std::type_identity_t to keep that
parameter non-deducible, so they only participate in overload resolution
when Signature is given explicitly. One accepts a free function pointer,
the other a pointer-to-member, enabling both fun<Ret(Args...)>(&free_fn)
and fun<Ret(Args...) cv>(&Class::method) to disambiguate overloads
without resorting to static_cast.

Files

 .../chaiscript/dispatchkit/register_function.hpp   | 37 ++++++++++++
 unittests/compiled_tests.cpp                       | 68 ++++++++++++++++++++++
 2 files changed, 105 insertions(+)

Closes #612

Triggered by @lefticus.