Start adding pyftmerge tool to merge fonts
[merge] Implement a coule more tables
[merge] Implement a few more straightforward tables
[merge] Implement OS/2 and cmap

Works for basic font mergin now.  Next: GSUB/GPOS/GDEF!
[merge] Whitespace
[merge] Implement --verbose and --timing
[merge] Remove unused code
[merge] Pass Merger down to merge() methods
[merge] Clean up imports
[merge] Minor
[merge] Implement GDEF

Only if GSUB/GPOS were so simple too!
[merge] Drop glyf hints

Since we don't renumber functions or CVTs, we have to drop hinting
for now.
[merge] Expand glyf components before merging

Such that components are converted to glyph names before remapping.
[merge] Minor
[merge] Use reduce more often
Support loading broken Coverage tables

Hit by Google Fonts's font "Cabin" for example and a handful
of others.
[subset] Handle inexistent feature / lookup lists
Allow None for globalSubrs in T2CharString
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.