tools: Fixes, Improvements to Opcodes/Code/Types

[marioevz]

Changes Included

• Add len to the Code class
• Allow Opcode's data portion or stack elements to be hex strings
• Adds a counter to the storage type, which allows to automatically store a value to the next slot in an internal counter, and return the slot number.

Storage counter

Tests can now use the following pattern:

bytecode = Op.SSTORE(storage.store_next(expected_value), Op.CALLDATALOAD(0))
bytecode += Op.SSTORE(storage.store_next(expected_value_2), Op.CALLDATALOAD(1))

And storage will keep track of the next storage slot to use, keeping code clean, and preventing overwriting a verification value in a loop for example.

[spencer-tb]

LGTM! :D