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/yul/src/parser/mod.rs

@@ -1,14 +1,14 @@
 //! The YUL code block.
 
+use crate::lexer::error::Error as LexerError;
+use crate::lexer::token::Token;
+use crate::lexer::Lexer;
+
 pub mod error;
 pub mod identifier;
 pub mod statement;
 pub mod r#type;
 
-use crate::lexer::error::Error as LexerError;
-use crate::lexer::token::Token;
-use crate::lexer::Lexer;
-
 /// Returns the `token` value if it is `Some(_)`, otherwise takes the next token from the `stream`.
 pub fn take_or_next(mut token: Option<Token>, lexer: &mut Lexer) -> Result<Token, LexerError> {
     match token.take() {