runtime: switch to gcWork abstraction

This converts the garbage collector from directly manipulating work
buffers to using the new gcWork abstraction.

The previous management of work buffers was rather ad hoc.  As a
result, switching to the gcWork abstraction changes many details of
work buffer management.

If greyobject fills a work buffer, it can now pull from work.partial
in addition to work.empty.

Previously, gcDrain started with a partial or empty work buffer and
fetched an empty work buffer if it filled its current buffer (in
greyobject).  Now, gcDrain starts with a full work buffer and fetches
an partial or empty work buffer if it fills its current buffer (in
greyobject).  The original behavior was bad because gcDrain would
immediately drop the empty work buffer returned by greyobject and
fetch a full work buffer, which greyobject was likely to immediately
overflow, fetching another empty work buffer, etc.  The new behavior
isn't great at the start because greyobject is likely to immediately
overflow the full buffer, but the steady-state behavior should be more
stable.  Both before and after this change, gcDrain fetches a full
work buffer if it drains its current buffer.  Basically all of these
choices are bad; the right answer is to use a dual work buffer scheme.

Previously, shade always fetched a work buffer (though usually from
m.currentwbuf), even if the object was already marked.  Now it only
fetches a work buffer if it actually greys an object.

Change-Id: I8b880ed660eb63135236fa5d5678f0c1c041881f
Reviewed-on: https://go-review.googlesource.com/5232Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>