Need ability to expose an R table to MetaR statements

[fac2003]

Inside an R script, it would be nice to be able to preview a table created in R code with preview table. We need three things to make this happen:

1. The identifier to export as a table must be annotated to indicate that the symbol need to be exported and that it contains a data.table object. Exporting will create a FutureTable instance.
2. The exported FutureTable instance must be visible in the scope available to MetaR statements located after the export.
3. We need to determine the columns of the exported table. These are needed to make design time previews work as usual. Perhaps the export annotation could write a small file with the columns and files. Coupled with instant refresh and loading of table columns from the file, this could work.