Support for the foreach looping construct. Foreach is an idiom that
allows for iterating over elements in a collection, without the use of an
explicit loop counter. This package in particular is intended to be used
for its return value, rather than for its side effects. In that sense, it
is similar to the standard lapply function, but doesn't require the
evaluation of a function. Using foreach without side effects also
facilitates executing the loop in parallel.
