`Indexer::index_tx` can have a default impl in the trait.

[evanlinjin]

          I think this is a good idea. Also `index_tx` can have a default impl in the trait.

Originally posted by @LLFourn in bitcoindevkit/bdk#1084 (comment)

[vladimirfomene]

@evanlinjin as discussed, I'm taking this up.

[LLFourn]

See bitcoindevkit/bdk#1176 (comment) why I no longer think this is a good idea.