String which has long decimal part makes invalid fixed_t

[MinoruSekine]

Describe the bug

• String which has long decimal part (e.g. "3.1415926535") makes invalid fixed_t

To Reproduce Steps to reproduce the behavior:

1. Define fixed_t typed value like as fixed_t kPi("3.1415926535");
2. It makes fixed_t which has invalid value

Expected behavior

• It makes closest value to pi which can be expressed

• Environment (please complete the following information):

  • OS: macOS 10.15
  • Compiler: g++ sym-linked to clang++

Additional context

• 195 will needs this fix

[MinoruSekine]

This addition to test occurs this issue.

[MinoruSekine]

Necessary solutions will be classified into some parts.

• Integer part is larger than max (e.g. "1234567890123.456")
• Integer part is smaller than min (e.g. "-1234567890123.456")
• The value is in capable range, but it has too much long decimal part (e.g. "3.1415926535")

[MinoruSekine]

FromStringToDecimalIntImpl() doesn't check integral overflow by long string. If cut too long string before conversion, it have to recognize "it is too long or not".

[MinoruSekine]

Without cutting too long string before conversion, codes like below may recognize overflow will be occurred or not.

std::numeric_limits<T>::max() / next_digit < current_value