Add 'pstore_value_write'

Couple the knowledge how to write a value to an extent to the value
itself.

Signed-off-by: Jussi Virtanen <jussi.k.virtanen@gmail.com>
Add 'pstore_value_format'

Couple the knowledge how to format a value to text to the value itself.

Signed-off-by: Jussi Virtanen <jussi.k.virtanen@gmail.com>
Add 'pstore_value_string'

Refrain from accessing the structure internals directly in anticipation
of introducing other value types in addition to strings.

Signed-off-by: Jussi Virtanen <jussi.k.virtanen@gmail.com>
Merge pull request #25 from jvirtanen/features/value-types

Prepare to support multiple value types