Add API method to compact the version map

[mehertz]

The version map is modelled as a linked list of versions. For symbols with a large number of versions, retrieving old versions means traversing the list which can be IO intensive (e.g. see #198)

To resolve this, we can compact multiple version keys into a single key. There is a method to do this:

lib._nvs.version_store._compact_version_map(<SYMBOL NAME>)

We would like to:

• Make this method public. It is private, which implies we may change it.

We might consider:

• Invoke the public method when calling any modifying operator (update, append, write and their batch equivalents). This should be controlled via a keyword argument.