Use released simplicity-lang

[jhfrontz]

I think this can now be { version = "0.3.1" } now, right?

https://github.com/BlockstreamResearch/simfony/blob/45c34f92543e9610a0c79916aaa919c97b0211ae/Cargo.toml#L29

[uncomputable]

We need the updates from BlockstreamResearch/rust-simplicity#289 to fix a major bug