Also buffer IFD reads

[feefladder]

Builds on #76, but also computes ifd size beforehand and fetches all of it. Only really beneficial for non-cogs.

The same:

• calculate IFD size = ifd_entry_byte_count * tag_count
• load into EndianAwareReader
• read the rest of the IFD using the EndianAwareReader

[kylebarron]

If this builds on #76 we can come back to discuss this after we resolve #76 and #78

[feefladder]

Also, this was just a thought I had. We can also close it, because it's mostly only useful for non-cog tiffs that have their IFDs not in the prefetch range, which could well be solved by a simple buffering reader. (I sense some resistance to #76, but that one is more important)