Allow using java keywords as solidity function names

[iikirilov]

What does this PR do?

Prepend the violating function name with "_" such that a solidity function called double will generate a java method called _double. This fixes #658

I am open to making this more extensible if required. For example, one could configure their own sanitizing string and specify whether it should be appended or prepended. An issue was opened on the JavaPoet github square/javapoet#690 - looks like we will have to implement ourselves in the future.

Where should the reviewer start?

SolidityFunctionWrapper

Why is it needed?

Because Solidity contract writers should not need to worry about Java keywords.

[codecov]

Codecov Report

Merging #776 into master will decrease coverage by 0.14%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #776      +/-   ##
============================================
- Coverage     77.38%   77.24%   -0.15%     
+ Complexity     1851     1850       -1     
============================================
  Files           241      241              
  Lines          6814     6816       +2     
  Branches       1012     1013       +1     
============================================
- Hits           5273     5265       -8     
- Misses         1291     1301      +10     
  Partials        250      250

Impacted Files	Coverage Δ	Complexity Δ
...ava/org/web3j/codegen/SolidityFunctionWrapper.java	93.98% <100%> (+0.01%)	123 <0> (+1)	⬆️
...n/java/org/web3j/protocol/core/filters/Filter.java	53.12% <0%> (-14.07%)	8% <0%> (-2%)
...org/web3j/protocol/websocket/WebSocketService.java	85.26% <0%> (-1.06%)	50% <0%> (-1%)
core/src/main/java/org/web3j/utils/Flowables.java	88% <0%> (+4%)	11% <0%> (+1%)	⬆️

---

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 6160282...e35745a. Read the comment docs.