Add one-arg method Duplicated(x)

[mcabbott]

Instead of writing

dx = make_zero(x)
xdx = Duplicated(x, dx)

this lets you write

xdx = Duplicated(x)

and later get dx = xdx.dval out when you need it.

Should there also be DuplicatedNoNeed(x), and perhaps BatchDuplicated(x, n::Int)?

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.22%. Comparing base (037dfed) to head (9e26b67).
Report is 238 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2118      +/-   ##
==========================================
+ Coverage   67.50%   70.22%   +2.71%     
==========================================
  Files          31       42      +11     
  Lines       12668    15784    +3116     
==========================================
+ Hits         8552    11084    +2532     
- Misses       4116     4700     +584     

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