Rename `EraSegWits` to `EraBlockBody`

[lehins]

Also:

• Rename TxSeq type family to BlockBody
• Add mkBasicBlockBody :: BlockBody, that would create an empty block body
• Add a lens txSeqBlockBodyL :: Lens' (BlockBody era) (StrictSeq (Tx era))
• Deprecate fromTxSeq and toTxSeq in favor of txSeqBlockBodyL
• Create hashBlockBody and deprecate hashTxSeq in its favor
• Stop exporting unsafe TxSeq' constructor from Cardano.Ledger.Shelley.BlockChain, by creating an internal module (similarly to how it is done in Alonzo)
• Rename Cardano.Ledger.Shelley.BlockChain to Cardano.Ledger.Shelley.BlockBody
• Rename Cardano.Ledger.Alonzo.TxSeq to Cardano.Ledger.Alonzo.BlockBody
• Rename ShelleyTxSeq and AlonzoTxSeq to ShelleyBlockBody and AlonzoBlockBody respectively. Also all of the fields will need to be renamed using the convention established in the ledger codebase