Use ordered hash table (bimap) of pointers for block txs.

[evoskuil]

This will improve validation by providing constant time lookup of outputs within a block.

Must deal properly with deserialization of an invalid block that contains duplicate transactions. Normally this is handled in validation (vs deserialization).

Expose txs via an iterator.

[evoskuil]

Also tx inputs and outputs.

[evoskuil]

Obsolete.