Add support for translations from .po and .mo files using rspolib

[tronical]

The rspolib feature adds RSPoLibTranslator and re-exports rspolib.

The plural rules parser was originally written by Olivier Goffart olivier.goffart@slint.dev.

[tronical]

As a side note: I had to do the edition upgrade for compilation, but I haven't done any other "upgrades". I'd be happy to do those, for example explicit features and using dep:foo, if desired. Similarly, I didn't change the version number but am happy to adjust in this PR if desired.

[ogoffart]

From an API point of view, I wonder if this should belong in a rspolib_01 module.

So we'd do:

tr::set_translator!(tr::rspolib_01::RSPoLibTranslator::try_from(
    tr::rspolib_01::rspolib::pofile("....po").unwrap()
).unwrap()));

But I get that this is a bit long to use tr::rspolib_01::rspolib
But the point is trying to keep it separate.

Another idea would be to hide the rspolib API entirely, and have an API like so:

fn load_po_file(path: Path) -> impl Translator
fn po_data(data: &[u8]) -> impl Translator
fn load_mo_file(path: Path) -> impl Translator
fn mo_data(data: &[u8]) -> impl Translator

[ogoffart]

Why is the MSRV so high?

[tronical]

From the last failed build attempt in the CI:

error: package `half v2.6.0` cannot be built because it requires rustc 1.81 or newer, while the currently active rustc version is 1.74.1

[ogoffart]

I'm wondering how to do the dynamic language change. We would have to call set_translator but that takes ownership. Meaning we need to always reload the file.

Perhaps the trick here is to implement the Translator trait for Arc

[tronical]

are you concerned that switching say between de, fr, and back to de, the de mo file is parsed twice?

I agree that Arc would fix it, but it's an interesting tradeoff to make (memory vs performance over a rare operation).