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/llvm-context/src/target_machine/mod.rs

@@ -1,12 +1,12 @@
 //! The LLVM target machine.
 
-pub mod target;
-
 use crate::optimizer::settings::size_level::SizeLevel as OptimizerSettingsSizeLevel;
 use crate::optimizer::settings::Settings as OptimizerSettings;
 
 use self::target::Target;
 
+pub mod target;
+
 /// The LLVM target machine.
 #[derive(Debug)]
 pub struct TargetMachine {