ToHex was deprecated a couple years ago. The last remaining use
was in ToHexArray, which itself only had a single call site.

This just moves ToHexArray near its only remaining call site and
implements it using hexutil.Encode. This changes the default behaviour
of ToHexArray and with it the behaviour of eth_getProof. Previously we
encoded an empty slice as 0, now the empty slice is encoded as 0x.