Improve decode_ignored_any coverage

[DrewMcArthur]

related to @haydnv/destream#2, I've found that attempting to utilize decode_ignored_any wasn't working for me. After looking at the code, it seems like this implementation could have better coverage.

goals for this issue:

• Impl/create actions #3 adds CI with automated tests based on serde-rs/json
• adds a test suite for this function, to ensure it's able to deserialize correctly and efficiently
• reimplement it, see this pr in my fork
• reach feature parity with serde's implementation (of this function)
• once Impl/create actions #3 is merged, open up a PR for my fix/2-decode-ignored-any branch

i'll likely have some questions about how some aspects should be approached. i'll start with the test suite though, in order to have a benchmark of it working.

(I'm not sure if I can assign myself to this issue, but feel free to do that @haydnv)

[DrewMcArthur]

finished with the merge of #4 in v0.12.2 🎉