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/source.rs

@@ -0,0 +1,40 @@
+//! The `solc --standard-json` input source.
+
+use std::io::Read;
+use std::path::Path;
+
+use serde::Deserialize;
+use serde::Serialize;
+
+/// The `solc --standard-json` input source.
+#[derive(Debug, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct Source {
+    /// The source code file content.
+    pub content: String,
+}
+
+impl From<String> for Source {
+    fn from(content: String) -> Self {
+        Self { content }
+    }
+}
+
+impl TryFrom<&Path> for Source {
+    type Error = anyhow::Error;
+
+    fn try_from(path: &Path) -> Result<Self, Self::Error> {
+        let content = if path.to_string_lossy() == "-" {
+            let mut solidity_code = String::with_capacity(16384);
+            std::io::stdin()
+                .read_to_string(&mut solidity_code)
+                .map_err(|error| anyhow::anyhow!("<stdin> reading error: {}", error))?;
+            solidity_code
+        } else {
+            std::fs::read_to_string(path)
+                .map_err(|error| anyhow::anyhow!("File {:?} reading error: {}", path, error))?
+        };
+
+        Ok(Self { content })
+    }
+}