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.

• Fix Removing capacity defaults #80
• Fix Fix code coverage reports #81
• Fix Placement new constructors #83 (note: postponed to after 1.0. We might want a SliceRingBuffer at some point)
• Fix AllocRingbuffer can use Box instead of Vec #86
• Support custom allocators on nightly (note: not implemented, very hard to make a feature flag for. Just wait until it's stable)
• Check (and update if necessary) MSRV. Fix this in cargo.toml
• Check which (pedantic) clippy lints we might want to apply (disallow certain useful pedantic clippy lints.  #85)
• Switch to main instead of master (you need to do this @NULLx76, since you are the repo owner)
• Check compile time performance (0.3 seconds, seems plenty fast, also few places to improve)
• Check run time performance (See -20% on some tests due to removed bounds check #88)
• Fix Implement traits for VecDeque #90
• From implementations (See From implementations #91)
• Remove deprecated methods
• Fix coverage on VecDeque
• Fix docs, check perf against heapless, etc