Merge bitcoindevkit/bdk#1986: ci: replace grcov by cargo-llvm-cov

Merge bitcoindevkit/bdk#1986: ci: replace grcov by cargo-llvm-cov

066c4cd160f6d4c70409b299a173f31f93728fae ci: replace grcov by cargo-llvm-cov (nymius)

Pull request description:

  ### Description

  As explained in #1984 , I discovered some misreportings in the coverage information given by grcov.

  Fixes #1984

  ### Notes to the reviewers

  The discussion about the tool to use is open, but I'm adding this PR to showcase how a change from `grcov` to `cargo-llvm-cov` would look like and the improvements in reporting this change will bring. [Look the report in `coveralls`](https://coveralls.io/github/nymius/bdk?branch=ci/replace-grcov-by-cargo-llvm-cov) to get a better perspective of them.

  ### 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 +nightly fmt` and `cargo clippy` before committing

ACKs for top commit:
  ValuedMammal:
    ACK 066c4cd160f6d4c70409b299a173f31f93728fae
  oleonardolima:
    ACK 066c4cd160f6d4c70409b299a173f31f93728fae

Tree-SHA512: d237fcc36efb6a4d3ccfaa371c2c70fc18ee40b48ca48f1230c5df6dca093f57bc1096927ddb15f9cae59a4640d9e34bf3ef1a309037e21fe0e8348e132bf38d