Small optimizations to BlockLayout.
Small optimizations to BlockLayout.

* Remove `takeL/R 1` occurences by lastOL/headOL.

* Make BlockChain a OrdList newtype by removing the set of blocks.

Initially BlockChain contained both, a set for membership test
and a ordered list of blocks. The set is not used for any
performance sensitive lookups so we get rid of it.

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc