Added deprecated attribute API lint, minor incompatibility level

[epage]

Semver says deprecations are minor changes.

Reference: https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-deprecated-attribute

Will require separate checks for:

• items (struct, trait, fn, etc.)
  • struct, enum
  • union
  • trait
  • fn
  • const / static (GlobalValue)
  • other items?
• enum variants
• struct fields
• trait items
• macro definitions
• external blocks

[pksunkara]

@obi1kenobi It hasn't been implemented for union. You might want to separate it out from struct & enum in the issue description.

[obi1kenobi]

Done, thanks!