Allow parsing only the block header when the txs are not required

[psgreco]

Hal counterpart of stevenroose/hal-elements#2

[stevenroose]

I just notice that this is a breaking change, because BlockInfo contains more info than BlockHeaderInfo, and this change makes it only show BlockHeaderInfo now even for blocks. I think you should first try to regularly parse as a full block and if that errors try if you can parse as a header.

[psgreco]

You're right, pushed a new version that doesn't break compatibility