internal: switch from CLI to internal benchmarking

We have a CLI for benchmarking, but no one actually uses it it seems.
Let's try switching to "internal" benchmarks, implemented as rust tests.
They should be easier to "script" to automate tracking of perf
regressions.

crates/rust-analyzer/src/lib.rs

@@ -39,6 +39,9 @@ mod op_queue;
 pub mod lsp_ext;
 pub mod config;
 
+#[cfg(test)]
+mod benchmarks;
+
 use serde::de::DeserializeOwned;
 use std::fmt;