make 'CsubsetDT' callable from C in other packages

[lsilvest]

I'm writing a package (lsilvest/dtts.utils) that has functions that subset data.table.

These functions provide time series functionality to a data.table that has as first column a nanotime type (eddelbuettel/nanotime) and will allow pretty cool subsettings on intervals, alignment to a grid, etc.

For this I need efficient subsetting at C level and I wouldn't need to duplicate C code if CsubsetDT were exported. It's a one liner to export it (R_RegisterCCallable("data.table", "CsubsetDT", (DL_FUNC) &subsetDT);) and I have committed it in a fork that could be pulled (7c84115).

Any chance this would be possible?

[jangorecki]

Recent idea in #3736 seems very related, but it is still R interface.
If you are planning to call it from C mainly then it make sense to go even deeper, as you propose. Eventually not just this single function but we can try to formulate C API, that would be also a step towards #852.

[lsilvest]

Indeed my use cases for calling CsubsetDT are within loops at C level that construct a new data.table, but I could construct an index at C level and then the solution in #3736 would be satisfactory, even if not always optimal and a bit less flexible.

[jangorecki]

@lsilvest could you fill PR to address your request?