minor: enable macros for generic types

[ksil]

Love this project so much! This PR:

• Adds a default implementation for struct utest_type_deducer<T, false> so that custom structs in C++ with proper overloaded operators can still enjoy all the UTEST macros without error. Previously, one could not use macros like EXPECT_LT without custom template specializations of utest_type_deducer.
• Removed an unnecessary const_cast
• Changed the printing function to pass by reference. This may have some very small performance overhead for small primitives if C++ if the compiler doesn't optimize it away, but consistency for all types is probably nice.
• Changed UTEST_AUTO to be a const auto reference. This not only accepts both lvalues and rvalues (temporaries) but it also now allows utest macros to work for structs that are not copy constructible.

Discussion:
In a future PR, it could be nice to check if a struct is forgettable via the concept std::formattable available in C++23. Then one could call std::format on the provided struct instead of printing (object [pointer]) as I do by default for now for max compatibility.

[sheredom]

Thanks! PR looks ok assuming green CI.

[ksil]

Awesome! Let me know if you need anything else to merge

[sheredom]

Not against adding support for std::formattable, just would want a before/after compile time cost of including that header from C++23 onwards!