decoder: add Frame::fragments() and Frame::display_fragments()

[darkwater]

One cool possibility with defmt is a log viewer that can do more than just display static text, eg. change number representations on the fly. These two methods should make doing something like that a bit easier.

Copying from the docs I put on display_fragments():

---

Returns an iterator over the fragments of the message contained in this log frame.

Collecting this into a String will yield the same result as Self::display_message, but
this iterator will yield interpolated fragments on their own. For example, the log:

defmt::info!("foo = {}, bar = {}", 1, 2);

Will yield the following strings:

vec!["foo = ", "1", ", bar = ", "2"]

Note that nested fragments will not yield separately:

defmt::info!("foo = {}", Foo { bar: 1 });

Will yield:

vec!["foo = ", "Foo { bar: 1 }"]

This iterator yields the same fragments as Self::fragments, so you can zip them
together to get both representations.

---

This is quite limited in that it doesn't go into nested fragments, but could already enable some limited extra functionality with minimal effort.

Haven't tested on an actual project yet, intend to do that tomorrow.