usfm2perf without Proskomma using Proskomma internals

[mvahowe]

The plan here is to rip out the Proskomma parser and repurpose it as a USFM to PERF parser. In the short term this would remove the Proskomma dependency in Epitelete and other places. It also paves the way for supporting PERF input to Proskomma, which is needed for Xelah.