Magic `str()` method creation in cgen should be moved higher.

[JalonSolov]

Presently, if an object does not have an explicitly defined str() method, and a request is made to print said object, a "magic" str() method is created in cgen.

This process is fine as far as it goes, but the "magic" str() methods need to be created before any backend is called, so that all backends can use them.

As it stands now, the "magic" str() method creation has to be duplicated in every backend separately.

Recent issue about this in #25825

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[jorgeluismireles]

arrays magic method str is shown in comptime (along others), next code works:

fn array_methods[T](a []T) {
	mut methods := []string{}
	$for m in a.methods {
		methods << m.name
	}
	assert methods == ['join', 'free', 'str', 'sort_ignore_case', 'sort_by_len', 'join_lines']
}

strings := ['1','2']
assert strings.str() == "['1', '2']"
array_methods(strings)

But user's struct Foo{} magic method str does not exist in comptime $for m in Foo.methods. Should be available?

[JalonSolov]

That's one of the problems of it not existing until cgen time. The comptime stuff is generally decoded and firmed before cgen.

[kbkpbot]

I want to move all the functions related to auto_fn in cgen to before the checker. For example, in the builder, when the first_stage ends and it is about to enter the middle_stage (checker), check whether various types in the code have already defined the str() method. If not, generate the corresponding V program code, then parse it via parser.parse_text(), and finally send it all together to the checker for inspection. This ensures that after the checker, all str() methods are defined.

[JalonSolov]

Carefully, though... if the type is never printed (or used in string interpolation, etc.), then it doesn't actually need a str() method.

If a str() method is created for every type whether it is needed or not, there will need to be a way to remove the extra code... hopefully markused will handle this, but it will need to be tested to be sure.

[medvednikov]

Done in V2