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/mod.rs

@@ -1,15 +1,6 @@
-//! The Solidity compiler unit tests.
+//! The Solidity compiler tests.
 
 #![cfg(test)]
 
 mod cli;
-mod factory_dependency;
-mod ir_artifacts;
-mod libraries;
-mod messages;
-mod optimizer;
-mod remappings;
-mod runtime_code;
-mod unsupported_opcodes;
-
-pub(crate) use super::test_utils::*;
+mod unit;