Fix String.toInt near maximum safe integer

[shin4141]

Summary

• normalize each digit before adding it to the running total, avoiding a transient value above JavaScript's maximum safe integer
• add regressions for the last unaffected boundary, the first affected odd value, and the positive and negative maximum safe integers

Verification

• unit suite: 741 passed, 0 failed
• integration suite: 1022 passed, 0 failed
• git diff --check

Closes #134.

[robinheghan]

Thank you!