Make a way to run debug_asserts on a Conf structure

[cbeck88]

We should have something similar to clap::Command::debug_assert(),

See also this discussion: #5

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After pondering it for a while, I think that instead of exposing a special builder for this, we should just expose a proc macro attribute, so like

#[derive(Conf)]
#[conf(serde, test)]
pub struct MyStruct {
    ...
}

This would auto-generate a #[cfg(test)] #[test] fn conf_test_MyStruct() { ... }, so you don't have to type any boilerplate yourself, and any options we were going to stick on the builder, we can make proc-macro arguments instead.

Tests we can run include:

• Run clap::Command::debug_assert()
• Check that short flags, long flags, serde names, don't collide. (But allow whitelisting some or all collisions)
• Check if default_value actually parses without errors according to value_parser.
• Recursively test subcommands

This can make it more natural to do certain things. For example, testing the default_value with the value_parser should usually work. But, if the value_parser reads a file, the test may fail if the file isn't there, and it may create a burden to make the test actually pass. In that case you might like to have a way to skip that test.

With a proc-macro attribute to control the testing, you can put #[arg(test(skip_value_parser_default_value))] on the problematic field.
Without proc-macro support, you have to find an unambiguous way to identify the field to the builder, or you have to use a mix of the builder API and the proc-macro attribute API, which seems messier.