AdEx v5: Issue #382 Channel and Campaign

[elpiel]

This PR tackles some of the changes required for #382.

Changelog:

Introduces the new structs & enums for AdEx v5:

• primitives::campaign::{Campaign, Validators, Active, etc.}
• primitives::channel_v5::Channel (should replace the channel module which has the v4 Channel)
  • implemented fn id(&self) -> ChannelId
  • primitives::channel_v5::Nonce - a U256 with a string (de)serializatoin instead of hex
• Address - should replace all the instances of ValidatorId where the ValidatorId is used for an Address (token address, contract address, spender, earner, etc.)
• primitives::ToHex - trait for encoding values to 0x prefixed or non-prefixed Strings
• primitives::eth_checksum - now uses tiny_keccak
• Cargo - primitives:
  • Updated tiny_keccak to 2.0
  • Removed rust-crypto dependency
  • Updated num to 0.4 and use the feature num-bigint
  • Removed num-bigint dependency
• Cargo - validator_worker:
  • Updated num to 0.4