IDEA: Improve performance by early skipping in findGlobals()

[HenrikBengtsson]

I've just made globals::findGlobals(X) significantly faster when X is long and contain lots of elements with basic data types. The gist of this improvement is to skip elements "early" that cannot contain globals:

types <- unlist(lapply(expr, FUN = storage.mode), use.names = FALSE)
keep <- which(!(types %in% c("logical", "integer", "double", "complex", "character", "raw", "NULL")))
globals <- lapply(expr[keep], FUN = findGlobals, ...)

Idea

Can this approach be done efficiently also in a nested approach, e.g. when X is a list of lists? Can it be done efficiently using plain R code, or do we need to implement this is native code? If we go for native code, other parts of the package could also go native - there's a lot of "walking" around in structures going on.