chore: add placeholder tco feature

Author: jonathanpwang

crates/vm/Cargo.toml

@@ -68,6 +68,9 @@ basic-memory = []
 # turns on stark-backend debugger in all proofs
 stark-debug = []
 test-utils = ["openvm-stark-sdk"]
+# Tail call optimizations. This requires nightly for the `become` keyword (https://github.com/rust-lang/rust/pull/144232) which will likely be stabilized in Rust 1.90
+# However `become` may still lead to compiler panics instead of runtime panics, so `tco` will remain a separate feature.
+tco = []
 # performance features:
 mimalloc = ["openvm-stark-backend/mimalloc"]
 jemalloc = ["openvm-stark-backend/jemalloc"]