[amount] Add support for negative fee rates

Currently negative fee rates are not supported on archs of 64-bit or
more

src/amount.h

@@ -11,6 +11,7 @@
 #include <stdlib.h>
 #include <string>
 
+/** Amount in satoshis (Can be negative) */
 typedef int64_t CAmount;
 
 static const CAmount COIN = 100000000;
@@ -30,22 +31,24 @@ extern const std::string CURRENCY_UNIT;
 static const CAmount MAX_MONEY = 21000000 * COIN;
 inline bool MoneyRange(const CAmount& nValue) { return (nValue >= 0 && nValue <= MAX_MONEY); }
 
-/** Type-safe wrapper class for fee rates
- * (how much to pay based on transaction size)
+/**
+ * Fee rate in satoshis per kilobyte: CAmount / kB
  */
 class CFeeRate
 {
 private:
     CAmount nSatoshisPerK; // unit is satoshis-per-1,000-bytes
 public:
+    /** Fee rate of 0 satoshis per kB */
     CFeeRate() : nSatoshisPerK(0) { }
     explicit CFeeRate(const CAmount& _nSatoshisPerK): nSatoshisPerK(_nSatoshisPerK) { }
-    CFeeRate(const CAmount& nFeePaid, size_t nSize);
+    /** Constructor for a fee rate in satoshis per kB. The size in bytes must not exceed (2^63 - 1)*/
+    CFeeRate(const CAmount& nFeePaid, size_t nBytes);
     CFeeRate(const CFeeRate& other) { nSatoshisPerK = other.nSatoshisPerK; }
     /**
      * Return the fee in satoshis for the given size in bytes.
      */
-    CAmount GetFee(size_t size) const;
+    CAmount GetFee(size_t nBytes) const;
     /**
      * Return the fee in satoshis for a size of 1000 bytes
      */