Repatriate rust_decimal implementation

[Sytten]

While working on the sea-query/diesel bridge, I found out that rust_decimal implementations ToSql and FromSql are in the rust_decimal crate. I find that bit unintuitive so I was wondering if we were interested in having our own implementations and feature flag.

Another motivation is that the way they do feature flagging (rust_decimal/db-diesel2-mysql, rust_decimal/db-diesel2-postgres) makes it annoying to provide a single feature in depending crate. I guess you can use the ?, but there is a notorious bug with (rust-lang/cargo#10801) and I tend to avoid it.

[weiznich]

I generally try to avoid adding more integrations to diesel itself, as at least I personally do not have capacity to maintain much more code. If someone else maintains the code in the corresponding crate, that's from my personal point of view a "better" solution for the maintenance problem.

[Sytten]

Thats fair, it really is annoying that we can't create a bunch of community repositories to maintain those integrations without them being in the core or third party. Hopefully we see relaxation to the orphan rule at some point.