Bump various dependencies & edition 2021

[nbittich]

This change is 

[codecov[bot]]

Codecov Report

Merging #222 (0077f58) into master (d86d1e2) will decrease coverage by 0.55%. The diff coverage is 79.47%.

Impacted Files	Coverage Δ
src/types/mailbox.rs	21.42% <ø> (+1.42%)	:arrow_up:
src/types/name.rs	53.33% <31.81%> (-22.67%)	:arrow_down:
src/types/flag.rs	43.58% <43.58%> (ø)
src/types/fetch.rs	43.93% <61.11%> (+1.93%)	:arrow_up:
src/error.rs	16.66% <63.63%> (+14.00%)	:arrow_up:
src/types/capabilities.rs	31.57% <66.66%> (+8.04%)	:arrow_up:
src/client.rs	85.77% <85.21%> (-0.61%)	:arrow_down:
src/parse.rs	84.77% <91.80%> (-2.50%)	:arrow_down:
src/extensions/metadata.rs	86.39% <97.53%> (+26.39%)	:arrow_up:
... and 9 more

[jonhoo]

I did some other cleanup too, and ended up doing this as well as part of that, so these changes are now on master :+1: