provide correct 'all.equal' for all types; closes #46

[lsilvest]

Rather long because the 'all.equal' needed to be defined on all 4 types.

[codecov]

Codecov Report

Merging #63 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #63   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          13     13           
  Lines        1841   1900   +59     
=====================================
+ Hits         1841   1900   +59

Impacted Files	Coverage Δ
R/nanoduration.R	100% <100%> (ø)	⬆️
R/nanotime.R	100% <100%> (ø)	⬆️
R/nanoperiod.R	100% <100%> (ø)	⬆️
R/nanoival.R	100% <100%> (ø)	⬆️

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[eddelbuettel]

Looks good. Do we need all the isTRUE() around what looks like scalar values?

Edit: Right on. There are special cases (like these) where one needs it.