generate expression xml parse data from full xml parse data

[AshesITR]

Another 20% performance improvement by parsing the XML only once and creating the expression documents from the full document.

> system.time(get_source_expressions(test_file))
   user  system elapsed 
  2.099   0.001   2.099 
> devtools::load_all()
ℹ Loading lintr
> system.time(get_source_expressions(test_file))
   user  system elapsed 
  1.635   0.001   1.636