Add option to export model keys

[Grimeh]

I'm restructuring my project and have noticed that model keys are defined with pub(crate) visibility.
I can see the motivation for this choice and generally agree with it. However it would be helpful to have the ability to make them pub for access from sibling crates.

From a quick look (and my limited experience with proc macros) it seems like it wouldn't be too difficult to add a parameter to the native_db macro to enable something like:

#[native_model(...)]
#[native_db(export_keys = true)]
pub struct MyTable { ... }

Then pub(crate) enum #keys_enum_name from native_db.rs:59 could be changed to something like #keys_visibility enum #keys_enum_name.

native_db/native_db_macro/src/native_db.rs

Lines 59 to 61 in 50063a0

	pub(crate) enum #keys_enum_name {
	#(#keys_enum),*
	}

I'm going to make a fork to try this out for my own purposes, would you be keen on this change if I opened a PR?

[Grimeh]

You can see my proposed impl here: main...Grimeh:export_keys

I added a rudimentary "does this compile with export_keys = true" unit test, but it doesn't test the changed visibility. I'm not sure that's possible to do without creating a separate test crate, which seems like overkill for one or two tests.

I'll open a PR if it looks good after some testing.

[vincent-herlemont]

Thank you very much, @Grimeh that's a great idea! You can submit your PR.

[vincent-herlemont]

I am closing this issue for now because there is no short-term need in the roadmap, but it will be reopened if needed.

Note: However, a PR is always welcome.

[Grimeh]

Sorry for the delay, just wrapped up a contract that took most of my time.
I'll finally make that PR :)