Replace tracing with log

[emilk]

Why? Writing backends for log is so much nicer than for tracing, and we don't use any of tracings advanced features anyway.

For native apps:

-    tracing_subscriber::fmt::init();
+    env_logger::init();

For web apps:

-    tracing_wasm::set_as_global_default();
+    eframe::web::WebLogger::init(log::LevelFilter::Debug).ok();

For those of you who use tracing and want to see log events, just use tracing = { version = "0.1", features = ["log"] }