Add STENO_MAP_MARKER

[vjkoskela]

Create a new marker that allows specification of data and context as a map. The primary use case is to replace the dual list (key-list and value-list) representation which allows duplicate keys. Once implemented we should add the appropriate methods to the custom Logger and then convert DefaultLogBuilder to use those. This will save on data copying in the LogBuilder in a major way which currently maintains two maps and then creates two lists per map to log the data and context.