Finish library interface

[xermicus]

Currently, Solang has a library interface, but it is only used for tests. In the long run we should be able to use our library interface as a the ground truth for all tests, the solang binary and anything else. Feel free to comment and collect ideas here regarding the direction we want to go with this interface.

[seanyoung]

• The library interface is also used by the solang binary, but the library interface has far too many public items.
• The library interface is also used by the language server, so it needs the entire ast exposed (but not codegen)
• The solang binary may save intermediates like llvm ir, assembly of cfg. This interface could be much cleaner
• The solana interface is extra ugly because currently Solana creates on binary for all contracts - this will soon change

Depends on #964 - once that is implemented, this will be much simpler