Make Solang compiled contracts compatible with Anchor

[seanyoung]

Remaining work (who is working on it in parenthesis)

• [x] Borsh decoding (Lucas)
• [x] Allow function selector to be overriden with new selector, variable length on Solana. (Sean)
• [x] Convert IDL file to solidity interface file solana metadata --target solana [-o outputfile] foo.idl (Sean)
• [x] Use borsh encoding for function arguments calls with overriden selector (provide end to end IDL tests with this) (Sean)
• [x] Use borsh encoding for events (Lucas)
• [x] Use borsh encoding for function return values with overriden selector (Sean)
• [ ] Rewrite contract dispatch code in CFG
• [ ] Use borsh encoding for ALL function calls and returns values (requires all tests to be fixed)
• [ ] Do not pass destination/sender account via input, but via accounts
• [ ] Solang should produce one binary per contract, just like Anchor/Substrate/ewasm (Sean)
• [ ] Produce IDL for Solana contracts
• [ ] Write tests using anchor CLI - anchor itself will need work!
• [ ] Write PDA tests. The seed will have to be passed in manually. Should just work (needs tests)
• [ ] abi.encode() and abi.decode() should use borsh encoding
• [ ] Use borsh encoding for Insert::AssertFailure
• [ ] Update documentation for try ... catch

[LucasSte]

It is not on this list, but many instructions utilize ABI encoding and need to be modified. One example is Instr::AssertFailure. Its arguments are ABI encoded in emit. That must change if we migrate to Borsh on Solana.

[seanyoung]

Note that try .. catch also uses abi encoding/decoding but this functionality does not work on Solana anyway. This needs clearer compiler error messages and documentation.