General cleanup: added constant for block_size

[GreyCat]

• Moved "4096" to a top-level constant block_size. Use that block_size everywhere applicable.
• Removed padding from block, as it serves no purpose anyway: the only invocation of block has clear constraint size: block_size, thus block would always be exactly 4096 bytes, no matter what.

[hddscan]

I was using the "padding" field here "_parent._parent.header.padding"
Please rename it if you don't like the name. Removing it is not an option.

[cugu]

It might be useful for forensics as well. We could call it slack_space.

[GreyCat]

That's different padding, it's padding from block_header. This commit removes padding from block.

My idea for removing it is that it stays accessible anyway, no information is lost. If anyone wants to examine it, one can always use certain_block._io and read (and do something, i.e. parse) its contents. But it's pointless to just read it into memory "because we can", as it quickly eats up RAM without doing nothing. If we'd want to do a working FS driver from this spec, we'd better avoid wasting RAM like that ;)

[hddscan]

My mistake. No objections then

[GreyCat]

@hddscan Checked it again: I agree that padding in block_header certainy needs to be renamed to avoid confusion. It's really weird to apply any logic (i.e. switching) over a "padding", which is "scrap area without any information" by definition.