use safe arithmetic wherever gas calculations can cause U256 to overflow

[voith]

What was wrong?

Related to Issue #342

#342 introduced safe arithmetic for handling gas calculations that could overflow U256. There are several more places where gas calculations can cause U256 to overflow.

How was it fixed?

Used safe arithmetic wherever gas calculations can cause U256 to overflow.

Note:

The Yellow paper says that the "data" size is unlimited:

There are some precompiles that calculate gas cost based on the input data size. This is the reason why u256_safe_add and u256_safe_multiply have been modified to accept Uint as input.

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[codecov-commenter]

Codecov Report

Merging #344 (7678500) into master (dace4e5) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #344      +/-   ##
==========================================
+ Coverage   95.04%   95.10%   +0.06%     
==========================================
  Files          46       46              
  Lines        2322     2351      +29     
==========================================
+ Hits         2207     2236      +29     
  Misses        115      115              

Flag	Coverage Δ
unittests	95.10% <100.00%> (+0.06%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...c/ethereum/frontier/vm/instructions/environment.py	98.93% <100.00%> (+0.09%)	⬆️
src/ethereum/frontier/vm/instructions/keccak.py	100.00% <100.00%> (ø)
src/ethereum/frontier/vm/instructions/memory.py	100.00% <100.00%> (ø)
src/ethereum/frontier/vm/instructions/system.py	89.85% <100.00%> (+0.22%)	⬆️
...reum/frontier/vm/precompiled_contracts/identity.py	100.00% <100.00%> (ø)
...eum/frontier/vm/precompiled_contracts/ripemd160.py	100.00% <100.00%> (ø)
...hereum/frontier/vm/precompiled_contracts/sha256.py	100.00% <100.00%> (ø)
src/ethereum/utils/safe_arithmetic.py	73.68% <100.00%> (ø)

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