v0.4.0 -- multi-`fn` contracts, fix `pub`, and rename `invariant` to `double_check`

Changes

• BREAKING: The invariant block has now been renamed to double_check (issue #24). The README
  has been updated to explain why this library now diverges from its original
  inspiration in D.

Added

• Contracts now accept more than one fn definition, which is MUCH more ergonomic for public interfaces in structs and traits.
• A code of conduct (see CONTRIBUTING.md) adapted from the Contributor Covenant 1.4.

Fixed

• Visibility modifiers (pub and its variants) were not accepted before this point. Now they are.