Cargo.toml: Only enable mysql_backend on diesel, not the whole mysql feature

[mcronce]

Only the mysql_backend feature is required for the traits/types; mysql pulls in the full MySQL client machinery (including mysqlclient-sys, which pulls in openssl, etc)

This only benefits diesel_async users, as far as I can tell; diesel_async uses separate connection machinery (mysql_async, mysql_common), which is pure Rust and uses a more permissive license

I would guess that the same could be applied to db-diesel-postgres, as diesel does have a postgers_backend feature which looks like it's a similar subset of the postgres feature, but I couldn't test that as we're only a MySQL shop and didn't want to submit anything untested.

Note that this may be considered a breaking change. Downstream crates/applications that pull in rust_decimal's db-diesel-mysql feature but not diesel's mysql feature, and which need to actually connect to MySQL using diesel's sync connections, are currently relying on the transitive dependency and will experience build failures.

[Tony-Samuels]

I'd consider this breaking change acceptable, but I'd like to wait for @paupino to confirm he's also happy with it before approving.

[paupino]

Yep, I agree! This seems like a minor risk - and while I realize it breaks semantic versioning slightly, I think it is worthwhile.

[Tony-Samuels]

@mcronce , are you able to fix up the tests? They'll need dev dependencies now.

[mcronce]

Definitely - I think that'll fix it