Implement a dedicated int/float parser for XML (schema) values in lxml.objectify.

This disables support for "_" in numbers, which are allowed by Python but not by XMLSchema.
Wee keep a few additional literals, such as "+NaN", simply because they shouldn't hurt.

See https://mail.python.org/archives/list/lxml@python.org/thread/6F7VIDKWZTJ6LB6VOX6IJNNWICYHFPNR/