feat: add tests for explicit/stable tag->u16 conversion

[arthurpaulino]

This PR adds tests that enforce stability for Tag -> u16 conversions.

Closes #1141 with a solution that's simpler than the manual re-implementation of conversion functions.

[huitseeker]

The conversion functions can be automatically derived through macros, we already have that in place:

lurk-rs/lurk-macros/src/lib.rs

Lines 476 to 531 in 3e86743

	/// This macro derives an impl of TryFrom<foo> for an enum type T with `#[repr(foo)]`.
	///
	/// # Example
	/// ```
	/// use lurk_macros::TryFromRepr;
	///
	/// #[derive(TryFromRepr)]
	/// #[repr(u8)]
	/// enum Foo {
	/// Bar = 0,
	/// Baz
	/// }
	/// ```
	///
	/// This will derive the natural impl that compares the input representation type to
	/// the automatic conversions of each variant into that representation type.
	#[proc_macro_derive(TryFromRepr)]
	pub fn derive_try_from_repr(input: TokenStream) -> TokenStream {
	let ast = parse_macro_input!(input as DeriveInput);
	let res_ty = get_type_from_attrs(&ast.attrs, "repr");
	let name = &ast.ident;
	let variants = match ast.data {
	Data::Enum(ref variants) => variants
	.variants
	.iter()
	.map(|v| &v.ident)
	.collect::<Vec<_>>(),
	Data::Struct(_) | Data::Union(_) => {
	panic!("#[derive(TryFromRepr)] is only defined for enums")
	}
	};
	match res_ty {
	Err(e) => {
	// If no explicit repr were given for us, we can't pursue
	panic!("TryFromRepr macro requires a repr parameter, which couldn't be parsed: {e:?}");
	}
	Ok(ty) => {
	let match_arms = try_from_match_arms(name, &variants, &ty);
	let name_str = name.to_string();
	quote! {
	impl std::convert::TryFrom<#ty> for #name {
	type Error = anyhow::Error;
	fn try_from(v: #ty) -> Result<Self, <Self as TryFrom<#ty>>::Error> {
	match v {
	#match_arms
	_ => Err(anyhow::anyhow!("invalid variant for enum {}", #name_str)),
	}
	}
	}
	}
	}
	}
	.into()
	}

[arthurpaulino]

@huitseeker the idea is to have hardcoded values that have to be changed by hand if someone changes the order of enum variants. See #1141, which was created after #1140 implemented a fix for the order swap introduced in #1133

[arthurpaulino]

In other words, we want to counter-balance the automation we already have with (healthy) friction that #1133 didn't have (i.e. the need to explicitly change the tests if the tag enums are changed).

[huitseeker]

This makes sense to me