Add a common crate (#75)

* Add a barebones common crate

* Refactor some code into the common crate

* Add a `ResolverApi` interface.

This commit adds a `ResolverApi` trait to the `format` crate that can be
implemented by any type that can act as a resolver. A resolver is able
to provide information on the chain state. This chain state could be
fresh or it could be cached (which is something that we will do in a
future PR).

This cleans up our crate graph so that `format` is not depending on the
node interactions crate for the `EthereumNode` trait.

* Cleanup the blocking executor

crates/common/src/iterators/mod.rs

@@ -0,0 +1,3 @@
+mod files_with_extension_iterator;
+
+pub use files_with_extension_iterator::*;