Generic ERC-20 token wrapper, sDai support

[ghost]

Base contract to wrap-and-supply an ERC-20 token, or withdraw-and-unwrap. Includes support for Dai -> sDai wrapping.

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[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -2.47% ⚠️

Comparison is base (551c497) 56.16% compared to head (14e5ad1) 53.70%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #182      +/-   ##
==========================================
- Coverage   56.16%   53.70%   -2.47%     
==========================================
  Files          26       28       +2     
  Lines         981     1026      +45     
  Branches      125      130       +5     
==========================================
  Hits          551      551              
- Misses        430      475      +45     

Files Changed	Coverage Δ
contracts/misc/token-wrapper/BaseTokenWrapper.sol	0.00% <0.00%> (ø)
...acts/misc/token-wrapper/SavingsDaiTokenWrapper.sol	0.00% <0.00%> (ø)

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