Currently we track the per-G GC assist balance as two monotonically
increasing values: the bytes allocated by the G this cycle (gcalloc)
and the scan work performed by the G this cycle (gcscanwork). The
assist balance is hence assistRatio*gcalloc - gcscanwork.

This works, but has two important downsides:

1) It requires floating-point math to figure out if a G is in debt or
   not. This makes it inappropriate to check for assist debt in the
   hot path of mallocgc, so we only do this when a G allocates a new
   span. As a result, Gs can operate "in the red", leading to
   under-assist and extended GC cycle length.

2) Revising the assist ratio during a GC cycle can lead to an "assist
   burst". If you think of plotting the scan work performed versus
   heaps size, the assist ratio controls the slope of this line.
   However, in the current system, the target line always passes
   through 0 at the heap size that triggered GC, so if the runtime
   increases the assist ratio, there has to be a potentially large
   assist to jump from the current amount of scan work up to the new
   target scan work for the current heap size.

This commit replaces this approach with directly tracking the GC
assist balance in terms of allocation credit bytes. Allocating N bytes
simply decreases this by N and assisting raises it by the amount of
scan work performed divided by the assist ratio (to get back to
bytes).

This will make it cheap to figure out if a G is in debt, which will
let us efficiently check if an assist is necessary *before* performing
an allocation and hence keep Gs "in the black".

This also fixes assist bursts because the assist ratio is now in terms
of *remaining* work, rather than work from the beginning of the GC
cycle. Hence, the plot of scan work versus heap size becomes
continuous: we can revise the slope, but this slope always starts from
where we are right now, rather than where we were at the beginning of
the cycle.

Change-Id: Ia821c5f07f8a433e8da7f195b52adfedd58bdf2c
Reviewed-on: https://go-review.googlesource.com/15408Reviewed-by: Rick Hudson <rlh@golang.org>