[rvm-research] Gen.virtualMemoryExhausted() does not work properly for GenCopy

Gen.colletionRequired() uses method virtualMemoryExhausted() as one
condition to trigger GC. This means GenCopy checks if (nursery + toSpace) *
COPY_EXPANSION larger than half available pages of toSpace, which in turn
leads to prematurely triggering minor GC. Instead, I think the correct
condition is  nursery * COPY_EXPANSION larger than available pages of
toSpace.

On the other hand, Gen.requiresFullHeapcollection() use
virtualMemoryExhausted() as one condition to determine whether triggering
full gc. It compares collection reserved pages with *toSpace* available
pages. However, till virtualMemoryExhausted() is called, the Mature has not
flipped. So It should compare reserved pages with *fromSpace* available
pages.

Also, I can't figure out why HALF of available mature space rather than
whole available mature space is checked.

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