Cleaned up auto decoding special case in std.conv.parse for integers

[JackStouffer]

Based on Andrei's advice in #4674, I cleaned up the code here.

Now even faster.

Built with ldc2 -O5 -release, code here.

# new
Total time: 10 ms, 730 μs, and 5 hnsecs

#current
Total time: 36 ms, 573 μs, and 4 hnsecs

#old
Total time: 181 ms, 850 μs, and 5 hnsecs

[andralex]

thx!

[JackStouffer]

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