Auto merge of #3242 - str4d:3058-key-encoding-refactor, r=str4d

Key encoding refactor

Includes code cherry-picked from the following upstream PRs:

- bitcoin/bitcoin#11372
  - Only the first three commits (the fourth commit depends on #2390)

Part of #3058.

src/miner.cpp

@@ -9,7 +9,6 @@
 #endif
 
 #include "amount.h"
-#include "base58.h"
 #include "chainparams.h"
 #include "consensus/consensus.h"
 #include "consensus/upgrades.h"
@@ -18,6 +17,7 @@
 #include "crypto/equihash.h"
 #endif
 #include "hash.h"
+#include "key_io.h"
 #include "main.h"
 #include "metrics.h"
 #include "net.h"