Minimal code splitting for TokenNetwork - using a library for pure functions

[palango]

Fixes #401

This PR splits the a first pure function out of the token network contract into an library. It also implements the necessary infrastructure for doing this, like deploying and linking the library into the token network contract code.

I did not yet pull all possible code into the library in order to make this PR smaller and easier to review. Once this is merged it should be easy to do this and reduce the contract code size further.

I also decided to only create a library for the token network contract now, as the code size gains seems small compared to the added complexity from deploying/linking for the other contracts look unworthy.

[palango]

@karlb This passes all but two tests now. I'm working on those.

If you want you can already give this a first look and provide some feedback.

[karlb]

Looks good so far! This will probably make changes to the etherscan verififaction necessary, too.

[codecov]

Codecov Report

Merging #1428 (7c48dce) into master (e268bd7) will increase coverage by 0.35%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1428      +/-   ##
==========================================
+ Coverage   80.27%   80.63%   +0.35%     
==========================================
  Files          21       22       +1     
  Lines        1516     1544      +28     
  Branches      187      191       +4     
==========================================
+ Hits         1217     1245      +28     
  Misses        253      253              
  Partials       46       46              

Impacted Files	Coverage Δ
raiden_contracts/constants.py	100.00% <100.00%> (ø)
raiden_contracts/deploy/contract_deployer.py	89.28% <100.00%> (+1.09%)	⬆️
raiden_contracts/deploy/contract_verifier.py	84.71% <100.00%> (+0.29%)	⬆️
raiden_contracts/utils/linking.py	100.00% <100.00%> (ø)

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[palango]

The etherscan verification are still WIP, but it can be pushed into a follow-up when necessary. The rest is good for review.