factor out solc JSON interface crate (#264)

The differential testing framework will make a second consumer. There
seems to be no re-usable Rust crate for this. But we already have
everything here, just needs a small refactor to make it fully re-usable.

- Mostly decouple the solc JSON-input-output interface types from the
`solidity` frontend crate
- Expose the JSON-input-output interface types in a dedicated crate

---------

Signed-off-by: Cyrill Leutwiler <bigcyrill@hotmail.com>

crates/solidity/src/tests/messages.rs

@@ -4,7 +4,7 @@
 
 use std::collections::BTreeMap;
 
-use crate::warning::Warning;
+use revive_solc_json_interface::warning::Warning;
 
 pub const ECRECOVER_TEST_SOURCE: &str = r#"
 // SPDX-License-Identifier: MIT