Remove ethcore::common re-export module (#2792)

* no longer export action_params

* remove transaction, header, receipt re-rexports from common

* remove env_info and builtins re-exports from common

* remove everything but util export from common

* replace common usages with util, remove module

* add a prelude module for ethcore-bigint

ethcore/src/json_tests/state.rs

@@ -17,6 +17,7 @@
 use super::test_common::*;
 use tests::helpers::*;
 use pod_state::{self, PodState};
+use log_entry::LogEntry;
 use ethereum;
 use ethjson;