Clean up tipsets

[timvermeulen]

Summary of changes
Changes introduced in this pull request:

• Normalization of the Tipset types to all use a lowercase s
• Tipset::new now doesn't clone block headers, and only clones Cids after the verification is done
• Tipset::is_empty is removed because tipsets are guaranteed to never be empty
• FullTipset::new now asserts that the vector of blocks isn't empty which we were already assuming in some places
• Other minor cleanup

[CLAassistant]

All committers have signed the CLA.

[ec2]

I had not noticed the casing issue... I'm surprised no ones OCD flared up haha. Good catch!

[austinabell]

Very nice!

So I think the inconsistent casing was due to the spec using both casings interchangably and the go implementations using TipSet... the inconsistency has bugged me and I prefer Tipset.. but I'll defer to other's opinions which is preferred for all

[dutterbutter]

LGTM 🔥🔥🔥