Solitidy ABI

Solidity ABI implementation in Rust.

This is still a work in progress:

• Type-safe encoding
• Runtime encoding
• Type-safe decoding
• Runtime decoding
• Parsing of Solidity ABI JSON output
• Type-safe encoding and decoding of Solidity ABI items
• Runtime encoding and decoding of Solidity ABI JSON items
• Procedural macros for Solidity ABI item generation
• Derive macro for Encode and Decode