Improved arithmetic overflow detection to fix #125

[josefblaha]

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Description

Fixes #125

This change improves arithmetic overflow detection when reading numbers in NumberTokenizer. The previous check doesn't work when the result happens to overflow to a value larger than the current one. Instead, I implemented two custom overflow checks for multiplication and addition.

Things to discuss:

• The two overflow checks TryMultiply and TryAdd seem reasonable to me, but maybe I forgot something. An alternative would be to play it safe, use a checked expression and catch OverflowException, thus trading some simplicity for slightly worse performance in corner cases.
• The change could be published to next 2.0.x bugfix release. I'm not sure what should be the target branch.

[FlorianRappl]

LGTM!

Thanks