Fix for 12837: Duration's get and individual unit getters are bug-prone.

Experience has shown that most people (at least some of who don't read
the documentation) seem to expect that get and the indivdual getters
return what total returns, causing bugs. So, this commit adds partial to
replace get (leaving get as deprecated) and deprecates the individual
unit getters, forcing folks to use total or partial explicitly. It's
likely that most code that will have to change because of these changes
is broken anyway, so this will catch bugs.