v0.1.7: Constant folding pass and fixes all over

Added
- Added a constant folding optimization pass to improve codegen. When variables fold
  to constant values, they are visible in the hover in the extension
- For Substrate and Solana, address literals can specified with their base58 notation, e.g.
  `address foo = address"5GBWmgdFAMqm8ZgAHGobqDqX6tjLxJhv53ygjNtaaAn3sjeZ";`
- Solana account storage implemented for ``bytes``, ``string``, and structs
- Implemented ``delete`` for Solana

Changed
- The Substrate target produces a single .contract file
- The Substrate target now uses the salt argument for seal\_instantiate()

Fixed
- Libraries are allowed to have constant variables
- Fixed ethereum abi encoding/decoding of structs and enums
- Solana now returns an error if account data is not large enough
- Fixed storage bytes push() and pop()
- Ewasm uses precompiles for keccak hashing
- Various ewasm fixes for Hyperledger Burrow

Signed-off-by: Sean Young <sean@mess.org>