Add write_ssz_files feature to beacon_chain crate

beacon_node/beacon_chain/Cargo.toml

@@ -4,6 +4,10 @@ version = "0.1.0"
 authors = ["Paul Hauner <paul@paulhauner.com>", "Age Manning <Age@AgeManning.com>"]
 edition = "2018"
 
+[features]
+
+write_ssz_files = []  # Writes debugging .ssz files to /tmp during block processing.
+
 [dependencies]
 eth2_config = { path = "../../eth2/utils/eth2_config" }
 merkle_proof = { path = "../../eth2/utils/merkle_proof" }

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -42,7 +42,7 @@ pub const GRAFFITI: &str = "sigp/lighthouse-0.0.0-prerelease";
 /// files in the temp directory.
 ///
 /// Only useful for testing.
-const WRITE_BLOCK_PROCESSING_SSZ: bool = true;
+const WRITE_BLOCK_PROCESSING_SSZ: bool = cfg!(feature = "write_ssz_files");
 
 #[derive(Debug, PartialEq)]
 pub enum BlockProcessingOutcome {