Reduce number of dependencies

[kellpossible]

It would be good to reduce the number of dependencies in the default build.

For example dashmap use in i18n-embed-fl could be replaced with a HashMap in a lock, etc, and only enabled using an optional feature if performance is really required by the end user at the cost of increased compile times. The chances are for almost every project, the compile time of dashmap far outweighs any benefits it brings to the runtime of the macro.

[kellpossible]

rust-embed should be an optional dependency for i18n-embed, in a sense, the crate has outgrown its utility as simply being a means to embed localizations with the I18nAssets trait and FileSystemAssets making it possible to load them at runtime. And it would be nice to use i18n-embed-fl without rust-embed dependency too.

[mrtryhard]

Chances are that lazy-static can be removed if std::sync::OnceLock is used instead. See the section in the README.md

[alerque]

Switching to OnceLock should be possible now, but doing so would bump the MSRV, correct?

[mrtryhard]

I couldn't find the current MSRV but I'm confident that it would probably bump higher the current MSRV to a minimum of 1.70.

[mrtryhard]

@alerque I've took time today to dig a bit for the current msrv, and I think it's fine to use OnceLock without it bumping the msrv.

root Cargo.toml -> depends on clap => msrv == 1.74
i18n-build -> depends on rust-embed => msrv  == 1.70
i18n-config -> depends on tinystr => msrv == 1.67.0 (OK, no lazy_static anyways)
i18n-embed -> depends on rust-embed => msrv == 1.70
i18n-embed-fl -> depends on rust-embed => msrv == 1.70

I am not yet 100% familiar with the dependencies but it does look OK.

[kellpossible]

Thanks @mrtryhard and @alerque ! I'd be very happy for a PR to remove lazy_static