Refactor tokens

[wangjj9219]

Refactor as follows:

1. unify the handling for ED for MutilCurrency(deposit, withdraw, transfer) between orml-tokens and the adapter for pallet-balances .
2. do some general abstract
3. add some calls to facilitate the separate use of orml-tokens

[codecov-commenter]

Codecov Report

Merging #565 (a795c73) into master (9ddd4da) will increase coverage by 1.40%.
The diff coverage is 96.22%.

@@            Coverage Diff             @@
##           master     #565      +/-   ##
==========================================
+ Coverage   72.68%   74.08%   +1.40%     
==========================================
  Files          76       76              
  Lines        5443     5741     +298     
==========================================
+ Hits         3956     4253     +297     
- Misses       1487     1488       +1     

Impacted Files	Coverage Δ
tokens/src/weights.rs	0.00% <0.00%> (ø)
tokens/src/lib.rs	85.94% <92.90%> (+3.37%)	⬆️
tokens/src/mock.rs	32.00% <100.00%> (-1.34%)	⬇️
tokens/src/tests.rs	100.00% <100.00%> (ø)
tokens/src/imbalances.rs	13.88% <0.00%> (-2.78%)	⬇️
xtokens/src/lib.rs	65.74% <0.00%> (ø)
authority/src/mock.rs	74.46% <0.00%> (+2.12%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9ddd4da...a795c73. Read the comment docs.