Version 1.9.0 supports MuPDF 1.9

Fixing bugs

* fixed invalid json format when images occur
* immunize against invalid invocation of doc.select() method (invalid
page number range, invalid document type)

PyMuPDF compatible with mupdf 1.8

Update performance documentation

Performance details on basic operation and text extraction.

PyMuPDF compatible with mupdf 1.7a

v1.7

Delete crashes_2.txt