Use marker trait StrictCodec to indicate strict one-to-one correspondance

[sorpaas]

This is related to #251. I'd like to put this out for discussions first to see if we like this approach before writing tests and other stuff.

This introduces a marker trait StrictCodec (together with derive(Strict)). Any types implementing StrictCodec are supposed to have strict one-to-one correspondence. This avoids accidentally introducing those non-strict types when it matters -- for example, when we need to handle user inputs and want to have peace of mind that we can freely decode and encode stuff.

[shawntabrizi]

Gav mentioned in Element chat that he would like to have strict enabled by default, and something like #[codec(permissive)] to give the current behavior.