Allow language servers to store Client as struct field

[ebkalderon]

Changed

• Change LspService::new() to accept an FnOnce(Client) -> T closure instead of an already initialized T value.
• Remove all &Client parameters from all LanguageServer trait methods.

These changes should enable users to initialize LanguageServer implementers with tower_lsp::Client conveniently stored as a struct field instead of having it passed into each callback method as an argument. Initialization would theoretically look like this:

#[tokio::main]
async fn main() {
    let stdin = tokio::io::stdin();
    let stdout = tokio::io::stdout();

    // let (service, messages) = LspService::new(Backend { ??? }); // Current
    let (service, messages) = LspService::new(|client| Backend { client }); // New
    Server::new(stdin, stdout)
        .interleave(messages)
        .serve(service)
        .await;
}

Additional resources required by the language server struct can be initialized ahead of time in main() and passed into the closure, if needed.

CC @Timmmm @icsaszar

Closes #189.

[Timmmm]

This seems much better, thanks!

[ebkalderon]

Awesome! In that case, I think I'll merge this pull request and consider #189 to be resolved. Feel free to open a new issue or submit a follow-up PR in case of further concerns or pain points you'd like to raise. Thanks again to all for the valuable input in previous discussions!