Add vctrs methods

[lionel-]

Part of r-lib/vctrs#1130

[codecov[bot]]

Codecov Report

Merging #247 into master will decrease coverage by 0.08%. The diff coverage is 86.11%.

@@            Coverage Diff             @@
##           master     #247      +/-   ##
==========================================
- Coverage   94.11%   94.02%   -0.09%     
==========================================
  Files          18       19       +1     
  Lines         900      920      +20     
==========================================
+ Hits          847      865      +18     
- Misses         53       55       +2     

Impacted Files	Coverage Δ
R/init.R	15.38% <0.00%> (-1.29%)	:arrow_down:
R/misc.R	100.00% <ø> (+3.33%)	:arrow_up:
R/mixed.R	100.00% <ø> (+2.08%)	:arrow_up:
R/tidyverse.R	88.57% <88.57%> (ø)

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[Enchufa2]

mixed_units, being a list, don't require any special treatment from the {vctrs} point of view, right? (Apart from adding "list" to the class, which we did already).

[lionel-]

Currently there is rudimentary support via fallbacks. I think #248 first needs to be addressed to get robust tidyverse support for mixed units (with or without vctrs methods).

[Enchufa2]

Merged, thanks!