codec module should never panic under any condition

[Gnurou]

Codecs should be hardened to the point where they cannot panic under any circumstance. This means no unwrap, no array indexes that could be out-of-bounds, etc. Any problem with the codec should return a specific error.

The problem is with detecting these panic conditions. There are a few features that could help:

• The missing_panics_doc, unwrap_used and expect_used clippy lints are great to warn about common panic points (the first one also warning upon panic! and assert!).
• The no_panic crate looks also helpful, but is limited to actual programs and requires some level of optimization to be really useful.