Address features by name

[behdad]

Currently there's a mismatch in the spec between how features and lookups are addressed:

• Lookups are always addressed by name. The name might be an integer.
• Features are addressed by their index. The index specified as the first token of feature lines is informational and ignored:

index <T> feature tag <T> list of lookups
The index is a number starting with 0 for the first entry. These numbers are ignored by the compiler but can be used as a reminder when the features are assigned to a script or language in the script table.

I suggest updating the spec to allow referring to features by name, which might be integers as well; or for backward compatibility, if a feature is referenced by an integer, that would be a feature index, but otherwise a feature name.

I'm going to implement something like this in my implementation, since I have the infrastructure set up already for lookups, and naming features is a rather useful feature.

WDYT?

[JelleBosmaMT]

Keep it simple. If you view the script and feature table in a text editor, numeric references keep everything tidy and compact: columns align, rows are short. No scrolling needed. We are not trying to create a programming language here. Lookups have labels because you have to be able to move them around, do searches to find them, identify their function. Not so for script and feature tables. Labels do not make anything easier.