[RFC] Remove -frtl-abstract-sequences in 4.5

[Arnaud Charlet]

> For critical things I did just what -frtl-abstract-sequences did (along 
> with some hand optimizing) and I've seen performance improvements of 50% 
> and more just by getting my working set of code to fit into the cache-size.
> 
> Once the feature is removed: Will there ever be any chance that the feature 
> will be re-implemented?

OK course, it's always possible to implement something (even from scratch)
at any point. If there's enough interest in this feature, then someone will
implement it "right" at some point, and keeping a broken version around
won't really help.

Arno