Merge bitcoindevkit/bdk#708: Change configs for source-base code coverage

0010ecd94aae49f389093cd3e8983c405049dbf5 Add badge to README (wszdexdrf)
690411722e2131f69a605a6f3b5e97911778e9ed Change configs for source-base code coverage (wszdexdrf)

Pull request description:

  ### Description

  This also changes the code coverage front end to coveralls instead of codecov, which had some issues with other changes in the PR. This will provide better and more accurate code coverage reports.

  ### Notes to the reviewers

  The tests run before generating the report are not exhaustive (not exhaustive earlier too, but I added as many as I could), and hence the report won't be 100% accurate.

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

ACKs for top commit:
  danielabrozzoni:
    ACK 0010ecd94aae49f389093cd3e8983c405049dbf5

Tree-SHA512: 04a21b7481b80287cf8a31276238b0a5958871310664363f54d53779bb2dda6f49198baaf3b5471667fa6131443b022aabd86f2e3b1cfcf5d4aacadf137a166e