Further simplify OTTableReader/Writer
Undo making subReader lazy

Need this for upcoming FeatureParams implementation.
[subset] Minor
[subset] Keep features with FeatureParams
Minor
Minor
Fix XML->ttf for fonts that have some missing anchors, etc

Eg. EBGaramond12-Regular.ttf.  It has a base anchor missing.
Dropping the empty table from XML changes the size of the base
anchor array, which will then result in an assertion because
all bases should have the same array length.

Fixes many other broken fonts too.
Fixup cmap lazy loading
Implement FeatureParams

All types of FeatureParams are correctly handled now.

The only thing not handled is broken fonts generated with the
old AFDKO that have their FeatureParams offset computed wrong.
I don't currently plan on handling those.

Fixes https://github.com/behdad/fonttools/issues/38