vctrs support

[krlmlr]

Part of #61.

Currently missing:

• [x] deprecation of as.hms() in favor of new as_hms()
• [x] vec_cast() for POSIXt (taking the tz attribute from the target)
• [x] as_hms.POSIXt should use vec_cast()

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@d77601d). Click here to learn what that means. The diff coverage is 82.08%.

@@            Coverage Diff            @@
##             master      #66   +/-   ##
=========================================
  Coverage          ?   92.57%           
=========================================
  Files             ?        9           
  Lines             ?      175           
  Branches          ?        0           
=========================================
  Hits              ?      162           
  Misses            ?       13           
  Partials          ?        0

Impacted Files	Coverage Δ
R/round.R	100% <100%> (ø)
R/parse.R	100% <100%> (ø)
R/arith.R	100% <100%> (ø)
R/coerce.R	60% <60%> (ø)
R/cast.R	77.27% <77.27%> (ø)
R/hms.R	95.91% <96%> (ø)

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[krlmlr]

@hadley: Would you like to take another look?