toml: fix custom to_toml for complex structs

[ttytm]

Quickfix for #19300

🤖 Generated by Copilot at 83c267e

This pull request enhances the toml module to support custom encoding of complex structs using the to_toml method. It also fixes the encoding of nested structs in arrays, and adds a test case for the new functionality.

🤖 Generated by Copilot at 83c267e

• Add support for custom encoding of complex structs using the to_toml method (link, link)

• Implement the to_toml method for the Example struct in vlib/toml/tests/encode_and_decode_test.v and assert that it matches the output of toml.encode (link)
• Modify the encode_struct function in vlib/toml/toml.v to check for nested structs in arrays and use the to_toml method if available, or the encode_struct function otherwise (link)
• Handle the special cases of Date, Time, and DateTime structs, which are already encoded by the toml module (link)

[spytheman]

Good work.

[spytheman]

Hm, the output of this does not seem right:

import toml

struct Problem {
        x int
}

struct Example {
        arr []Problem
}

pub fn (problem Problem) to_toml() string {
        return 'a problem'
}

fn test_encoding_of_struct_with_a_field_that_has_itself_custom_to_toml_method() {
        example := Example{ arr: [Problem{}, Problem{}] }
        dump(toml.encode(example))
}

It produces:

[toml_with_custom_method_test.v:19] toml.encode(example): arr = [
  { x = 0 },
  { x = 0 }
]

i.e this pub fn (problem Problem) to_toml() string { is not called, even though it is defined.

[ttytm]

Thanks for the review. Indeed. It should be fixed.