Road to 1.0

[jdonszelmann]

This is a (currently non-exhaustive) list of problems that need fixing before a release of ringbuffer version 1.0. Because upgrading to 1.0 means we cannot change our API anymore, we need to think carefully about what we want to fix before that.

• [x] Fix #80
• [x] Fix #81
• [x] Fix #83 (note: postponed to after 1.0. We might want a SliceRingBuffer at some point)
• [x] Fix #86
• [x] Support custom allocators on nightly (note: not implemented, very hard to make a feature flag for. Just wait until it's stable)
• [x] Check (and update if necessary) MSRV. Fix this in cargo.toml
• [x] Check which (pedantic) clippy lints we might want to apply (#85)
• [x] Switch to main instead of master (you need to do this @NULLx76, since you are the repo owner)
• [x] Check compile time performance (0.3 seconds, seems plenty fast, also few places to improve)
• [x] Check run time performance (See #88)
• [x] Fix #90
• [x] From implementations (See #91)
• [x] Remove deprecated methods
• [x] Fix coverage on VecDeque
• [ ] Fix docs, check perf against heapless, etc