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/polkavm/context/function/mod.rs

@@ -1,12 +1,5 @@
 //! The LLVM IR generator function.
 
-pub mod declaration;
-pub mod intrinsics;
-pub mod llvm_runtime;
-pub mod r#return;
-pub mod runtime;
-pub mod yul_data;
-
 use std::collections::HashMap;
 
 use inkwell::debug_info::AsDIScope;
@@ -20,6 +13,13 @@ use self::declaration::Declaration;
 use self::r#return::Return;
 use self::yul_data::YulData;
 
+pub mod declaration;
+pub mod intrinsics;
+pub mod llvm_runtime;
+pub mod r#return;
+pub mod runtime;
+pub mod yul_data;
+
 /// The LLVM IR generator function.
 #[derive(Debug)]
 pub struct Function<'ctx> {