Add std::chrono::time_point constructor overloads and tests

[yalagadapavankumar]

Add C++ std::chrono::time_point constructors in Rust using overload! macro.

• Default constructor: Creates time point at the clock's epoch.
• Constructor from milliseconds: Creates time point from duration in milliseconds.
• Converting constructor: Creates time point from another time point that uses seconds.

1. Write Rust wrappers for that C++ type, for 3-5 different overloaded method names:
   • Create a new file named after your type in overloading-macros/cpp-overload-test/src/stdcpp
   • Wrap constructors first, writing tests to make sure the correct constructor is called
   • Explain any potential compatibility issues in a Limitations doc section
   • Explain usability issues in an Ergonomics doc section
   • Check that you have implemented all the overloads for each method name, or documented why you can't
   • Push your changes and wait for a review
2. Add a separate example with some failing overloads
   • Create a new file named after your type in overloading-macros/cpp-overload-test/examples
   • Wrap accessor/modifier methods
   • Explain how they fail in an Incompatibilities doc section
   • Explain how the overload failures could be resolved in a Workarounds doc section
   • Optional: If there's a C++ feature with no equivalent in Rust, explain that in an Inexpressible Overloads doc section
   • Push your changes and wait for a review
3. Update the overload macro compatibility table Markdown doc
   • If you haven't checked some kinds of overloads, put a ？in those squares
   • Push your changes and wait for a review

[yalagadapavankumar]

Hi teor, i thought using just checklist is more convenient, as it makes easier to what completed.

Do you think it's better to include whole PR template?

[teor2345]

for the October round, and as far as I can see, they lead to the October channels.

this is the link representing as text which is not for October channel, which is exits in all 3 rust projects, for this project Extend Rust stdlib custom allocator support, they didn't provide zulip properly, so people believe this text is zulip channel for this project.

This is now fixed, and if people accidentally introduce themselves somewhere else, someone from the community will move their post to the right channel.

[teor2345]

Sorry, the instructions weren't clear, we want to see 3-5 different overloaded function names, wrapping all the overloads for each name.

I have updated the PR template and the checklist at the top of this PR with the new wording.

You can choose whatever overloaded functions you like, but I suggest:

• https://en.cppreference.com/cpp/chrono/time_point/time_point_cast
• https://en.cppreference.com/cpp/chrono/time_point/round

[teor2345]

This specific type alias makes the code harder to read, because it hides that the type is milliseconds.
Instead, delete this alias, and use the milliseconds type directly.

Please also rename TestTimePoint so it is clear it is in milliseconds.

[teor2345]

As above, hiding the units makes the type error-prone to use.

Suggested change

	pub struct StdTimePoint(*mut c_void, PhantomData<()>);
	pub struct StdMillisecondsTimePoint(*mut c_void, PhantomData<()>);

[teor2345]

The i64 to long long type conversion may fail on some platforms. Instead, use the Rust type that is guaranteed to be a long long on all platforms:
https://doc.rust-lang.org/std/ffi/type.c_longlong.html

Please make this change throughout this file.