Add streaming/XML fragment support

[oubiwann]

We're going to have to use a different XML library, since erlsom doesn't seem to have streaming support. Currently, the most promising seems to be:

• https://github.com/processone/fast_xml

Tasks/features:

• Add dependency
• Add streaming wrapper
• Ensure streaming works with memory-mapped files
• Examine XML data model (how close to erlsom)
• Swap out existing erlsom-based lxml code for fast_xml
• Add backwards-compatible datamodel wrapper

Related tickets:

• Add support for memory mapped files #14

This is a follow-up ticket from the closed issue:

• Add wrapper support for erlsom sax mode #13