v0.8.0

Release Notes: 0.8.0 (2020-10-25)

• The #[derive(BitfieldSpecifier)] now allows an amount of variants that is not a power of two via
  the new attribute #[bits = N] where N is the bit width of the deriving enum. (Thanks @lkolbly)
• Add Specifier implementations for u8, u16, u32, u64 and u128.
• The #[bitfield] macro now additionally generates getters for expected failures just as it already
  does for the various setters. For a field a the new fail-safe getter is called a_or_err and returns
  a Result.
• Silence repetitive dead_code warnings originating from generated #[bitfield] getters and setters.
• Improve error span information in a few use cases.
• Cleaned up backend code for modular_bitfield crate and its generated code.