RFR: JDK-8307356: Metaspace: simplify BinList handling

[Thomas Stuefe]

In preparation for Lilliput, I'd like to simplify BinList handling a bit.

BinList are a data structure that stores small blocks that had been prematurely deallocated for re-use. Due to the way BinList is implemented we need the blocks to be at least two words. That causes ripples all across metaspace, including testing code. We can get rid of this complexity by shrinking a Block to just one word. 

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Details:

A BinList block before: 
``` Block { Block* next; size_t size; }```
A BinList block now: 
``` Block { Block* next; }```

We don't need to store the block size, since all blocks in a single sub list have the same size. In fact, we used the block size only for verification.

That means that for 64-bit gross- and net-allocation size in Metaspace are the same (see `get_raw_word_size_for_requested_word_size`), which in turn will make writing tests that try to predict metaspace usage based on allocation pattern a lot less onerous to write.

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Commit messages:
 - BinList changes

Changes: https://git.openjdk.org/jdk/pull/13776/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13776&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307356
  Stats: 142 lines in 11 files changed: 35 ins; 62 del; 45 mod
  Patch: https://git.openjdk.org/jdk/pull/13776.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13776/head:pull/13776

PR: https://git.openjdk.org/jdk/pull/13776