Swap the actual and expected strings in self.assertEqual() calls so that
the diffs are shown correctly.

See inline comments and the example in the test.

See inline comments.

This seems to be what VOLT does.

If any of the input glyphs is a glyph class, we need to explode it into
multiple statements since feature file syntax does not support classes
in ligature substitutions.

VOLT ranges are GID-based, while feature files ranges are glyph
names-based, so the two are not compatible and we should expand VOLT
ranges.

Since we need to map VOLT glyph names to font glyph names and groups
reference glyphs.

If only some of the glyphs are ligatures or marks, then VOLT makes it a
mark to base lookup and uses the anchor of the first ligature component
(in case of ligatures).

In VOLT group definitions can be ordered in any order, but in FEA a
group must be defined before being referenced in another group. We had
some code to sort the groups to fix this, but it was broken in several
ways. We now use the more appropriate TopologicalSorter(), and nested
enums when getting what groups a group references.

Without this fix we would output invalid fea code like:

    pos [[a b] [c d] [e f g h i j k l] [m n o p q]]' lookup lookup_target space;

Rename variables to be less cryptic.

In FEA syntax aalt is not allowed to have script or language statements,
but in OpenType and VOLT it is a regular feature like any other. We now
emit a warning and use only lookups from first script and language.

No functional change. Prepares for next commit.

Using inline syntax results in very inefficient lookups, with each
statement going into a subtable. Some fonts get too big lookups and even
fail to compile. Using explicit lookup blocks for the chained lookups
fixes most of such undesired subtable breaks.

VOLT might skip the padding spaces, but feaLib.ast will assert if not
equal to 4.

VOLT ignores it unless it is a single substitution lookup, so we now do
the same instead of raising.

Add __main__.py CLI that takes a MS VOLT project and compiles it to
font tables. Layout tables are currently converted first to feature
files and compiled with feaLib. Support for other tables (e.g. cmap)
will be added later.

At some point direct compilation of layout tables (without converting to
feature files first) might be added.