This creates `Block.Ref`, a reference to a `Block` which may or may not
be part of a `Page`. `Block.Ref` is `Releasable` and closing it is a
noop if the `Block` is part of a `Page`, but if it is "free floating"
then closing the `Block.Ref` will close the block.
It also modified `ExpressionEvaluator` to return a `Block.Ref` instead
of a `Block` - so you tend to work with `ExpressionEvaluator`s like
this:
```
try (Block.Ref ref = eval.eval(page)) {
return ref.block().doStuff();
}
```
This should make it *much* easier to release the memory from `Block`s
built by `ExpressionEvaluator`s.
This change is mostly mechanical, introducing the new signature for
`ExpressionEvaluator`. In a follow up change I'll modify the tests to
make sure we're correctly using it to close pages.
I did think about changing `ExpressionEvaluator` to add a method telling
you if the block that it returns must be closed or not. This would have
been more difficult to work with, and, ultimately, limiting.
Specifically, it is possible for an `ExpressionEvaluator` to *sometimes*
return a free floating block and other times return one that is
contained in a `Page`. Imagine `mv_concat` - it returns the block it
receives if the block doesn't have multivalued fields. Otherwise it
concats things. If that block happens to come directly out of the
`Page`, then `mv_concat` will sometimes produce free floating blocks and
sometimes not.
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