new measurev function

[tdhock]

This is to address #4998 who suggested a new function measurev (analogous to measure, like setkey and setkeyv).

>      (two.iris = data.table(datasets::iris)[c(1,150)])
   Sepal.Length Sepal.Width Petal.Length Petal.Width   Species
1:          5.1         3.5          1.4         0.2    setosa
2:          5.9         3.0          5.1         1.8 virginica
>      melt(two.iris, measure.vars = measure(part, dim, sep="."))
     Species  part    dim value
1:    setosa Sepal Length   5.1
2: virginica Sepal Length   5.9
3:    setosa Sepal  Width   3.5
4: virginica Sepal  Width   3.0
5:    setosa Petal Length   1.4
6: virginica Petal Length   5.1
7:    setosa Petal  Width   0.2
8: virginica Petal  Width   1.8
>      my.list = list(part=NULL, dim=NULL)
>      melt(two.iris, measure.vars=measurev(my.list, sep="."))
     Species  part    dim value
1:    setosa Sepal Length   5.1
2: virginica Sepal Length   5.9
3:    setosa Sepal  Width   3.5
4: virginica Sepal  Width   3.0
5:    setosa Petal Length   1.4
6: virginica Petal Length   5.1
7:    setosa Petal  Width   0.2
8: virginica Petal  Width   1.8

Internally I refactored measure so that is it just parsing the ... and then calling measurev.

[codecov]

Codecov Report

Merging #5022 (1d6eb1b) into master (c179b5e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #5022   +/-   ##
=======================================
  Coverage   99.47%   99.47%           
=======================================
  Files          75       75           
  Lines       14789    14807   +18     
=======================================
+ Hits        14711    14729   +18     
  Misses         78       78           

Impacted Files	Coverage Δ
R/fmelt.R	100.00% <100.00%> (ø)

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[tdhock]

wow that was fast thanks Matt!