Release v0.3.0

• Added cleaners and weak pointers
• Added the derive macros for Trace and Finalize
• Added no-std support
• Improved documentation
• Removed invalid Ccs and reworked Cc::new_cyclic to use weak pointers
• Countless small fixes and improvements

Internal changes:

• Use Iai-Callgrind instead of Iai for running benchmarks in CI