Separate compilation and linker phases (#376)

Separate between compilation and linker phases to allow deploy time
linking and back-porting era compiler changes to fix #91. Unlinked
contract binaries (caused by missing libraries or missing factory
dependencies in turn) are emitted as raw ELF object.

Few drive by fixes:
- #98
- A compiler panic on missing libraries definitions.
- Fixes some incosistent type forwarding in JSON output (empty string
vs. null object).
- Remove the unused fallback for size optimization setting.
- Remove the broken `--lvm-ir`  mode.
- CI workflow fixes.

---------

Signed-off-by: Cyrill Leutwiler <bigcyrill@hotmail.com>
Signed-off-by: xermicus <bigcyrill@hotmail.com>
Signed-off-by: xermicus <cyrill@parity.io>

crates/resolc/src/tests/unit/factory_dependency.rs

@@ -0,0 +1,70 @@
+//! The Solidity compiler unit tests for factory dependencies.
+
+use crate::test_utils::{build_solidity, sources};
+
+#[test]
+fn default() {
+    let caller_code = r#"
+// SPDX-License-Identifier: MIT
+
+pragma solidity >=0.4.16;
+
+import "./callable.sol";
+
+contract Main {
+    function main() external returns(uint256) {
+        Callable callable = new Callable();
+
+        callable.set(10);
+        return callable.get();
+    }
+}"#;
+
+    let callee_code = r#"
+// SPDX-License-Identifier: MIT
+
+pragma solidity >=0.4.16;
+
+contract Callable {
+    uint256 value;
+
+    function set(uint256 x) external {
+        value = x;
+    }
+
+    function get() external view returns(uint256) {
+        return value;
+    }
+}"#;
+
+    let output = build_solidity(sources(&[
+        ("main.sol", caller_code),
+        ("callable.sol", callee_code),
+    ]))
+    .unwrap();
+
+    assert_eq!(
+        output
+            .contracts
+            .get("main.sol")
+            .expect("Missing file `main.sol`")
+            .get("Main")
+            .expect("Missing contract `main.sol:Main`")
+            .factory_dependencies
+            .len(),
+        1,
+        "Expected 1 factory dependency in `main.sol:Main`"
+    );
+    assert_eq!(
+        output
+            .contracts
+            .get("callable.sol")
+            .expect("Missing file `callable.sol`")
+            .get("Callable")
+            .expect("Missing contract `callable.sol:Callable`")
+            .factory_dependencies
+            .len(),
+        0,
+        "Expected 0 factory dependencies in `callable.sol:Callable`"
+    );
+}