message: chore: add `make gen` to generate files
diff: +FLATC := flatc
  +CFBC := cfbc
  VERBOSE := $(if ${CI},--verbose,)
  test:
  @@ -50,6 +52,19 @@ security-audit:
  docker:
  docker build -f docker/hub/Dockerfile -t nervos/ckb:latest .
  -.PHONY: build prod prod-test docker
  -.PHONY: fmt test clippy proto doc doc-deps check stats
  +GEN_FILES := protocol/src/protocol_generated.rs protocol/src/protocol_generated_verifier.rs
  +gen: ${GEN_FILES}
  +gen-clean:
  + rm -f ${GEN_FILES}
  +
  +%_generated.rs: %.fbs
  + $(FLATC) -r -o $(shell dirname $@) $<
  +
  +%_generated_verifier.rs: %.fbs
  + $(FLATC) -b --schema -o $(shell dirname $@) $<
  + $(CFBC) -o $(shell dirname $@) $*.bfbs
  + rm -f $*.bfbs $*_builder.rs
  +
  +.PHONY: build prod prod-test docker gen gen-clean
  +.PHONY: fmt test clippy doc doc-deps check stats
  .PHONY: ci info security-audit
  