Add DIY logger

[notmandatory]

          Should we open an issue to introduce some sort of diy logger as ldk does?

Originally posted by @danielabrozzoni in #1204 (comment)

[notmandatory]

Added this issue as a placeholder. It's not part of the user API so can do it at any time.

[LLFourn]

My take is that our architecture doesn't require logging. We no longer have any long running processes occurring in bdk itself. You could do logging in the blockchain crates for example but that can be decided on a crate by crate basis.

[notmandatory]

Closing this issue since we only depend on the log crate in examples, not in any of the lib crates.

[LLFourn]

(might be a nice idea to remove log from the examples. You can just use println! in examples)