SI-6412 alleviates leaks in toolboxes, attempt #2

Turns importer caches into fully weak hash maps, and also applies
manual cleanup to toolboxes every time they are used.

It's not enough, because reflection-mem-typecheck test is still leaking
at a rate of ~100kb per typecheck, but it's much better than it was before.
We'll fix the rest later, after 2.10.0-final.

For more information, see https://issues.scala-lang.org/browse/SI-6412 and
http://groups.google.com/group/scala-internals/browse_thread/thread/eabcf3d406dab8b2

In comparison with https://github.com/scala/scala/commit/b403c1d,
the original commit that implemented the fix, this one doesn't crash tests.
The problem with the original commit was that it called tryFixup() before
updating the cache, leading to stack overflows.

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