Handle and warn about out-of-order Coverage table entries

Apparently the SIL Gentium fonts have wrong sorting in Coverage
tables.  That's broken by the spec, but shapers still work with
such fonts.  So, we warn when reading, and when writing, such
out-of-order coverage tables.
Fix parsing of hinting instructions that end in a PUSHB

Sounds useless, but MReifutaiM-Regular.ttf seems to have that, and the
parsing back from XML was failing because we expected a token whereas
after dropping whitespace there wasn't any left.