Replace deprecated methods in config

This MR replaces the deprecated Config::set method by using ConfigBuilder::add_source on the previously built configuration.

Also, since rustc is single-threaded, lazy_static! + RwLock has been replaced by thread_local! + RefCell.

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Default features of config have also been disabled, removing 32 unused crate dependencies from the final build.

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Note that the write_setting currently overrides the previous configuration rather than merging it (hence, the fix name before the commit name).

rr_frontend/.cargo/config.toml

@@ -15,7 +15,6 @@ rustflags = [
 
     # Rust warnings
     "-Adead_code",
-    "-Adeprecated",
     "-Aunused_imports",
     "-Aunused_mut",
     "-Aunused_variables",