Deprecate public constructors for `Base{16/32/64}` implementations

[05nelsonm]

In preparation for version 3.0.0 modality changes, public constructors need to be deprecated in order to work up to a gradual DeprecationLevel of HIDDEN.

e.g.

@Deprecated("This constructor will be removed in a future release. Use Base16.Builder or Base16.Companion.Builder")
public constructor(config: Config): this(config, unused = null)

@Suppress("UNUSED_PARAMETER")
private constructor(config: Config, unused: Any?): super(config)

Additionally, make all EncoderDecoder abstract functions final as to preempt the future modality modification.