This way it can be more easily reused for blinded payment paths.

We'll similarly separate blinded path payments code into its own module.

Useful for blinded payment path construction.

We want a similar macro for reading TLV streams without a length prefix, so
rename this one to disambiguate.

Useful for when you want to use _init_and_read_len_prefixed_tlv_fields but there is no
length byte at the start of the TLV stream.

Also adds a util for general blinded hop creation to be reused for blinded
payment paths.

The previous name can be confused for the shared secret that the rho is derived
from.