Added coverage CI and badge

[LucaCappelletti94]

Closes issue #133

I may make more PRs in the future to extend the coverage, which at this time is at around 50% according to the following tarpaulin command:

cargo tarpaulin --engine=llvm --out xml --workspace --run-types AllTargets --run-types Doctests --all-features

Producing the following output:

|| Tested/Total Lines:
|| src/into_either.rs: 7/7 +0.00%
|| src/iterator.rs: 15/79 +0.00%
|| src/lib.rs: 143/238 +0.00%
|| src/serde_untagged.rs: 7/10 +0.00%
|| src/serde_untagged_optional.rs: 7/12 +0.00%
|| 
51.73% coverage, 179/346 lines covered, +0.00% change in coverage

Best,
Luca