Mimic the API of embedded-rust/itm and ultimately replace it

[tmplt]

The current API of itm_decodes leaves a lot to desire. The API of embedded-rust/itm is more proper and should be mimiced if we mean to replace it.

[tmplt]

As discussed in the Embedded Rust Matrix channel, itm-decode is queued to replace embedded-rust/itm following a Cortex-M team vote. The API of this crate will be refactored to better follow Rust API guidelines by copying some ideas from the existing itm crate, but also by implementing the API change discussed in #19. This work is done on the replace-itm-crate branch. When this is done (and presuming that the vote passes), a PR to embedded-rust/itm will be created. Following the merge, embedded-rust/itm v0.4.0 will be released and this crate will be deprecated.

[tmplt]

This has been handles as of rtic-scope/itm@3931431.