Add SegmentedList.shrink
Add SegmentedList.shrink

I was exploring std.zig.Tokenizer and wanted to compare performance of
arrays, SegmentedList and ArrayList and needed SegmentedList.shrink
to make the comparison "fair".

raulgrell/zig,zig-lang/zig,zig-lang/zig,raulgrell/zig,raulgrell/zig,raulgrell/zig,andrewrk/zig,raulgrell/zig,zig-lang/zig,andrewrk/zig