Add a const constructor for ChainSystem

[nicopap]

What problem does this solve or what need does it fill?

Currently, it is impossible to define system chains in a const variable such as:

fn sys1() -> Out {
}
fn sys2(In(out): In<Out>) {
}
const systems: ChainSystem<???> = ChainSystem::new(sys1, sys2);

Such pattern would allows proc macros such as bevy_mod_sysfail
to be much more flexible, for example allowing arbitrary system for
handling system output.

What solution would you like?

ChainSystem currently doesn't have a const constructor. What
need to be done to implement such a constructor is:

• Define a non-trait public const constructor for ChainSystem (pub const fn new(…) -> Self)
• That constructor, unlike the existing ones, must not rely on trait methods, but rather
  directly call const functions on concrete types/type parameters.

What alternative(s) have you considered?

The current workaround used by bevy_mod_sysfail is to not allow arbitrary
system error handlers, and just insert the error handling code into the system
marked with the attribute macro.

Directly using ChainSystem would allow simplifying the proc macro code as well.

Additional context

This was discussed on discord.