return invisibly
allowing tools:rstudio to remain in search path

Had weird errors when we removed this, even after re-attaching
it, in particular relating to all the frame functionality not
working anymore (e.g. search panel, package viewer).
now re-loading packages with library and not attach

Also added support function is.loaded_package, and a bunch of
other cleanup
substantial test re-org, and is.loaded_package tests
interim commit before changing strategy

Too many issues trying to handle attaches and detaches,
will try to shim attach/detach/etc. with trace
in process of converting to shims

also added S4 object to keep track of changes
adding hooks for new zero env

still work in progress, and definitely not tested
new zero env strategy mostly settled

but not tested at all, major changes to file from
before so that we can rely on tracing rather than
shimming functions