Optimized several methods:

- Argument.EnsureEnumIsValid will cache the presence of the flags attribute since reflecting on this is slow.
- StringExtensions.SplitQualified will range check the options argument for speed.
- GifImage<T> has two improvements:
-- DataBuffer.Iterator - a new class for efficient setting of data in the buffer.
-- The hashing function now takes an array parameter, allowing the complier to do more inlining magic.