Rollup merge of #149061 - jdonszelmann:fixed-size-encoding-assert, r=oli-obk

debug-assert FixedSizeEncoding invariant

Something like this? It asserts during encoding that for that type, decoding 0 would give the default.
Preferably, I'd either somehow statically/in const assert it once, instead of every time, but I see no easy way to do so. It'd require us to iterate all types that implement the trait or something. Let me know what you think

No types currently violate this invariant.

r? `@oli-obk`

Author: matthiaskrgr