Use const generics in tls_codec array.rs

[franziskuskiefer]

Seems like something which would nicely be replaced with const generics, whenever you're ready to bump MSRV to 1.51.

Originally posted by @tarcieri in #56 (comment)

[tarcieri]

Note that this workspace is using resolver = "2", which is necessary to avoid activation of std-dependent features in no_std crates when using build-dependencies/dev-dependencies with common dependencies.

That carries with it an MSRV of 1.51 anyway, so I'd say it would make sense to go ahead and change this.