fixed log tests that were failing because gas calculations were resulting in U256 to overflow

[voith]

What was wrong?

Some gas calculations were causing U256 to overflow.

How was it fixed?

• Implemented u256_safe_add and u256_safe_multiply that add and multiuply respectively and raise OutOfGasError if the result exceeds U256.MAX_VALUE.
• Uncommented tests that were failing because of the above-mentioned error.

Note

I haven't applied the new safe arithmetic methods in other places where gas calculation could possibly overflow. I can do this in a separate PR.

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[codecov-commenter]

Codecov Report

Merging #342 (8d823dc) into master (232c42b) will decrease coverage by 0.15%.
The diff coverage is 84.84%.

@@            Coverage Diff             @@
##           master     #342      +/-   ##
==========================================
- Coverage   95.19%   95.04%   -0.16%     
==========================================
  Files          45       46       +1     
  Lines        2291     2319      +28     
==========================================
+ Hits         2181     2204      +23     
- Misses        110      115       +5     

Flag	Coverage Δ
unittests	95.04% <84.84%> (-0.16%)	⬇️

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Impacted Files	Coverage Δ
src/ethereum/utils/safe_arithmetic.py	73.68% <73.68%> (ø)
src/ethereum/frontier/vm/gas.py	100.00% <100.00%> (ø)
src/ethereum/frontier/vm/instructions/log.py	100.00% <100.00%> (ø)

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