More effective implementation of MnemonicUtils.generateEntropy

[serso]

It doesn't manipulate with strings that much as the old version.
Instead it uses a BitSet.

Also, some static tests for MnemonicUtils.generateEntropy were added in
this commit.

[codecov]

Codecov Report

Merging #665 into release/4.0 will increase coverage by 0.25%.
The diff coverage is 80%.

@@                Coverage Diff                @@
##             release/4.0     #665      +/-   ##
=================================================
+ Coverage          76.76%   77.01%   +0.25%     
+ Complexity          1798     1795       -3     
=================================================
  Files                239      239              
  Lines               6713     6661      -52     
  Branches            1004      998       -6     
=================================================
- Hits                5153     5130      -23     
+ Misses              1304     1281      -23     
+ Partials             256      250       -6

Impacted Files	Coverage Δ	Complexity Δ
.../src/main/java/org/web3j/crypto/MnemonicUtils.java	78.3% <80%> (-0.6%)	34 <8> (+2)
...a/org/web3j/protocol/websocket/events/NewHead.java	13.33% <0%> (+0.83%)	2% <0%> (ø)	⬇️
...ava/org/web3j/codegen/SolidityFunctionWrapper.java	95.94% <0%> (+2.05%)	114% <0%> (-5%)	⬇️
core/src/main/java/org/web3j/tx/Contract.java	75.67% <0%> (+6.44%)	46% <0%> (ø)	⬇️
.../org/web3j/protocol/websocket/WebSocketClient.java	80% <0%> (+7.27%)	6% <0%> (ø)	⬇️

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