An ECKey is a composition of a private key (D), a public key (Q) and its
compression flag.
These functions gave the impression of serialization of this
composition, when really they only serialized `D`.
They have therefore been removed in favour of always using a sane
serialization format (WIF) that matches the needed behaviour.
If a user needs the previous functionality, simply use `privKey.D.*`
instead of `privKey.*`, as BigInteger supports `*Buffer/*Hex` functions
as expected.
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