Add @xw.cache decorator for UDFs

[stenci]

I was not able to tell Excel to use the repository code, so I edited the modules installed with pip from envs\xlwings\Lib\site-packages\xlwings\udfs.py and tested that the UDF import to Excel was working well.

When the import was working, I copied the code to the modules in the repository, committed and created the pull request.

I was not able to edit the xlwings.xlam file from the repository. In order for these changes to work, the following function must be added to the module utils.bas:

Function NewDictionary(ParamArray Args()) As Dictionary
  Dim I As Integer
  Set NewDictionary = New Dictionary
  For I = 0 To UBound(Args) Step 2
    NewDictionary.Add Args(I), Args(I + 1)
  Next I
End Function