Separate immutable contracts, only inherit as needed

contracts/staking/contracts/src/StakingProxy.sol

@@ -22,13 +22,15 @@ pragma experimental ABIEncoderV2;
 import "./libs/LibProxy.sol";
 import "./libs/LibSafeDowncast.sol";
 import "./immutable/MixinStorage.sol";
+import "./immutable/MixinConstants.sol";
 import "./interfaces/IStorageInit.sol";
 import "./interfaces/IStakingProxy.sol";
 
 
 contract StakingProxy is
     IStakingProxy,
-    MixinStorage
+    MixinStorage,
+    MixinConstants
 {
     using LibProxy for address;
     using LibSafeDowncast for uint256;

contracts/staking/contracts/src/immutable/MixinConstants.sol

@@ -18,28 +18,15 @@
 
 pragma solidity ^0.5.9;
 
-import "./MixinDeploymentConstants.sol";
 
+contract MixinConstants {
 
-contract MixinConstants is
-    MixinDeploymentConstants
-{
     // 100% in parts-per-million.
     uint32 constant internal PPM_DENOMINATOR = 10**6;
 
-    // The upper 16 bytes represent the pool id, so this would be pool id 1. See MixinStakinPool for more information.
-    bytes32 constant internal INITIAL_POOL_ID = 0x0000000000000000000000000000000100000000000000000000000000000000;
-
-    // The upper 16 bytes represent the pool id, so this would be an increment of 1. See MixinStakinPool for more information.
-    uint256 constant internal POOL_ID_INCREMENT_AMOUNT = 0x0000000000000000000000000000000100000000000000000000000000000000;
-
     bytes32 constant internal NIL_POOL_ID = 0x0000000000000000000000000000000000000000000000000000000000000000;
 
     address constant internal NIL_ADDRESS = 0x0000000000000000000000000000000000000000;
 
-    bytes32 constant internal UNKNOWN_STAKING_POOL_ID = 0x0000000000000000000000000000000000000000000000000000000000000000;
-
-    uint64 constant internal INITIAL_EPOCH = 0;
-
     uint256 constant internal MIN_TOKEN_VALUE = 10**18;
 }

contracts/staking/contracts/src/immutable/MixinStorage.sol

@@ -29,7 +29,6 @@ import "../libs/LibStakingRichErrors.sol";
 
 // solhint-disable max-states-count, no-empty-blocks
 contract MixinStorage is
-    MixinConstants,
     Authorizable
 {
     // address of staking contract

contracts/staking/contracts/src/stake/MixinStakeBalances.sol

@@ -21,11 +21,13 @@ pragma experimental ABIEncoderV2;
 
 import "@0x/contracts-utils/contracts/src/LibSafeMath.sol";
 import "../interfaces/IStructs.sol";
+import "../immutable/MixinDeploymentConstants.sol";
 import "./MixinStakeStorage.sol";
 
 
 contract MixinStakeBalances is
-    MixinStakeStorage
+    MixinStakeStorage,
+    MixinDeploymentConstants
 {
     using LibSafeMath for uint256;
 

contracts/staking/contracts/src/staking_pools/MixinCumulativeRewards.sol

@@ -22,10 +22,12 @@ pragma experimental ABIEncoderV2;
 import "@0x/contracts-utils/contracts/src/LibFractions.sol";
 import "@0x/contracts-utils/contracts/src/LibSafeMath.sol";
 import "../stake/MixinStakeBalances.sol";
+import "../immutable/MixinConstants.sol";
 
 
 contract MixinCumulativeRewards is
-    MixinStakeBalances
+    MixinStakeBalances,
+    MixinConstants
 {
     using LibSafeMath for uint256;
 

contracts/staking/contracts/src/sys/MixinParams.sol

@@ -20,13 +20,15 @@ pragma solidity ^0.5.9;
 
 import "@0x/contracts-utils/contracts/src/LibRichErrors.sol";
 import "../immutable/MixinStorage.sol";
+import "../immutable/MixinConstants.sol";
 import "../interfaces/IStakingEvents.sol";
 import "../libs/LibStakingRichErrors.sol";
 
 
 contract MixinParams is
     IStakingEvents,
-    MixinStorage
+    MixinStorage,
+    MixinConstants
 {
     /// @dev Set all configurable parameters at once.
     /// @param _epochDurationInSeconds Minimum seconds between epochs.