This makes the output of feaLib more compact, using a similar technique
as seems to be used by makeotf.
After this change, feaLib generates output that more similar to makeotf:
* For the test cases in `bug512.fea` and `bug463.fea`, feaLib now
generates the exact same output as makeotf v2.0.90.
* For the test cases in `GSUB_6.fea`, it is hard to say because makeotf
crashes on the test file; our test contains language constructs that
are valid according to the spec, but didn't yet get implemented by makeotf.
When commenting out those constructs, feaLib generates the exact same
output as makeotf v2.0.90.
* For the test cases in `feature_aalt.fea`, the output of feaLib is now
structually the same as the output of makeotf v2.0.90. However, two
lookups are in different order. feaLib's ordering reflects the order
of statements in the compiled input source; no idea why makeotf would
want to reverse the ordering. Since this ordering difference only
affects the _targets_ of chain substitutions, there is no semantic
difference.
Resolves https://github.com/behdad/fonttools/issues/512.
