to facilitate alternate List implementations, factor out
implementation-specific code from SimpleList subset/extraction methods
into low-level generics/methods; common code has been pushed up into List;
CompressedList could be refactored in a similar way but it for some
reason prefers coercing indices to ranges so needs some thought


git-svn-id: https://hedgehog.fhcrc.org/bioconductor/trunk/madman/Rpacks/IRanges@76967 bc3139a8-67e5-0310-9ffc-ced21a209358
Merge branch 'local-hedgehog'

Conflicts:
	DESCRIPTION
adding 'IntervalForest-class.R' to Collate list again
Merge branch 'master' of https://github.com/hcorrada/IRanges