ink! 3.0.0 RC 2

Version 3.0-rc2 (2020-10-22)

This is the 2nd release candidate for ink! 3.0.

On top of the changes introduced in the first release candidate for ink! 3.0 we introduced
the following improvements, new features and bug fixes:

• The ink_storage crate now comes with a new BinaryHeap data structure
  that has a very similar interface to the well known Rust standard library
  BinaryHeap. It features specific optimizations to reduce the storage reads
  and writes required for its operations.
• Fixed a bug with ink_storage::Lazy that corrupted the storage of
  other storage data structures if it was unused in a contract execution.
• The ink_storage::alloc::Box type now implements scale_info::TypeInfo which
  now allows it to be fully used inside other storage data structures such as
  ink_storage::collections::Vec. The missing of this implementation was
  considered a bug.
• The LazyHashMap low-level storage abstraction is now re-exported from within
  the ink_storage::lazy module and docs are inlined.
• Added note about the ink_core split into ink_env and ink_storage crates
  to the release notes of ink! 3.0-rc1.
• The Cargo.toml documentation now properly links to the one deployed at docs.rs.
  On top of that crate level documentation for the ink_allocator crate has been
  added.
• Add new ERC-20 example contract based on a trait implementation. Also modernized
  the old non-trait based ERC-20 example token contract.