Much needed TLC

[awestlake87]

WIP: aiming for 0.21 release - might cherry-pick changes into a 0.20.1 patch if it takes too long though

• Combine Runtime and ContextExt (Runtime is never used without ContextExt)
• Remove deprecated conversions
• Update clap
• Update syn
• Update tokio
• Update once_cell
• Update async-channel
• Intern strings Intern strings for performance #106
• Investigate new async library (https://github.com/wyfo/pyo3-async)
  • Needs more discussion
• Implement Rust <-> Python Channels? (with official Stream and Async Generator conversions) Mappings between Stream and Async Generators #17
  • Needs more discussion
  • pyo3-async may already have the necessary conversions for this
• Move 0.13-0.15 Migration Guides out of README (store it in a changelog maybe?)
  • Been so long at this point that it's probably just confusing to new users
• Touch up docs