Support MMR digest in block header (darwinia-network#358)

* move mmr digest from sytem to darwinia pallet

* fix compile

* using tuple instead of vec

* update the map hasher

* add positions for leaves and prepare for later get_proof and verify functions

* add docs and add get proof function

* add tests and refactor codes

* update substrate dep and add header-mmr tests in travis

* fix format issue

* travis says can not parse, try again

* update decl error, and keep using panic when encoutering error in on_finalize, related to #384

* remove miss typing \ which should be used in <pre> code

.maintain/ci/travis.local.sh

@@ -5,6 +5,7 @@ readonly TEST_CRATES=(
     'treasury'
     'eth-relay'
     'eth-backing'
+    'header-mmr'
 );
 
 function main() {

.travis.yml

@@ -29,30 +29,34 @@ jobs:
       env: RUST_TOOLCHAIN=nightly-2020-02-07 TARGET=wasm
       script: .maintain/ci/build_script.sh
 
-    - stage: Darwinia Test
+    - stage: Darwinia KTON Test
       env: RUST_TOOLCHAIN=nightly-2020-02-07 TARGET=native KTON
       script: .maintain/ci/darwinia_test_script.sh kton
 
-    - stage: Darwinia Test
+    - stage: Darwinia RING Test
       env: RUST_TOOLCHAIN=nightly-2020-02-07 TARGET=native RING
       script: .maintain/ci/darwinia_test_script.sh ring
 
-    - stage: Darwinia Test
+    - stage: Darwinia STAKING Test
       env: RUST_TOOLCHAIN=nightly-2020-02-07 TARGET=native STAKING
       script: .maintain/ci/darwinia_test_script.sh staking
 
-    - stage: Darwinia Test
+    - stage: Darwinia TREASURY Test
       env: RUST_TOOLCHAIN=nightly-2020-02-07 TARGET=native TREASURY
       script: .maintain/ci/darwinia_test_script.sh treasury
 
-    - stage: Darwinia Test
+    - stage: Darwinia ETHRELAY Test
       env: RUST_TOOLCHAIN=nightly-2020-02-07 TARGET=native ETHRELAY
       script: .maintain/ci/darwinia_test_script.sh eth-relay
 
-    - stage: Darwinia Test
+    - stage: Darwinia ETHBACKING Test
       env: RUST_TOOLCHAIN=nightly-2020-02-07 TARGET=native ETHBACKING
       script: .maintain/ci/darwinia_test_script.sh eth-backing
 
+    - stage: Darwinia HEADERMMR Test
+      env: RUST_TOOLCHAIN=nightly-2020-02-07 TARGET=native HEADERMMR
+      script: .maintain/ci/darwinia_test_script.sh header-mmr
+
     - stage: Darwinia Test
       env: RUST_TOOLCHAIN=nightly-2020-02-07 TARGET=native ETHOFFCHAIN
       script: .maintain/ci/darwinia_test_script.sh eth-offchain