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/solc-json-interface/src/standard_json/input/settings/metadata.rs

@@ -0,0 +1,24 @@
+//! The `solc --standard-json` input settings metadata.
+
+use serde::Deserialize;
+use serde::Serialize;
+
+use crate::standard_json::input::settings::metadata_hash::MetadataHash;
+
+/// The `solc --standard-json` input settings metadata.
+#[derive(Debug, Default, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct Metadata {
+    /// The bytecode hash mode.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub bytecode_hash: Option<MetadataHash>,
+}
+
+impl Metadata {
+    /// A shortcut constructor.
+    pub fn new(bytecode_hash: MetadataHash) -> Self {
+        Self {
+            bytecode_hash: Some(bytecode_hash),
+        }
+    }
+}