Don't require import of internal macros

Don't require import of internal macros

Commit f560320b introduced changes that require users of
`impl_writeable_tlv_based`/`impl_writeable_tlv_based_enum` to import
`_encode_varint_length_prefixed_tlv` and `alloc` separately.

Here, we take care of the necessary imports in
`_encode_varint_length_prefixed_tlv` itself, allowing users to just
import the `impl_writeable_tlv_based` variant they need.