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Special case chunk encoding for `dict` chunk store (#359)

* Add `values` method to `CustomMapping`

For doing a quick pass through the values within a `MutableMapping`, it
is helpful to have the `values` method. This will be needed in some of
the tests that we are adding. So add a quick implementation of `values`
in the `CustomMapping`.

* Test stores contain bytes-like data

* Add a test to check that all values are bytes

Currently this is a somewhat loose requirement that not all stores
enforce. However it is a useful check to have in some cases.
Particularly this is a useful constraint with in-memory stores where
there is a concern that the data in the store might be manipulated
externally due to ndarray views. The bytes instances don't have this
problem as they are immutable and own their data. While views can be
take onto bytes instances, they will be read-only views and thus are not
a concern.

For other stores that place their data in some other storage backend
(e.g. on disk), this is less of a concern. Also other stores may choose
to represent their data in other ways (e.g. LMDB with `memoryview`s).
Manipulating the loaded data from these stores is less of a concern
since as it doesn't affect their actually contents, only an in-memory
representation. In these cases, it may make sense to disable this test.

* Disable bytes value test for LMDBStore w/buffers

It's possible to request that `LMDBStore` return `buffer`s/`memoryview`s
instead of returning `bytes` for values. This is incredibly useful as
LMDB is memory-mapped. So this avoids a copy when accessing the data.
However this means it will fail `test_store_has_bytes_values`. Though
this is ok as noted previously since that is not a hard requirement of
stores. So we just disable that test for this `LMDBStore` case. The
other `LMDBStore` case returns `bytes` instances instead (copying the
data); so, it passes this test without issues.

* Have CustomMapp... (continued)

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