Implement string conversion methods

[Cat-Ion]

Both MultiwordInt and FixedPoint should have methods to convert them to strings, possibly with the usual floating-point modifiers for FixedPoint (i.e. precision and printing as scientific/engineering/...)

[dmitvitalii]

Hi. I want to take this issue.

[Cat-Ion]

Awesome! I think the best way to do this here would be to have to_string() methods on both classes that each return a std::string - std::to_string doesn't allow you to have options like the precision, and using a std::stringstream feels a bit heavy.

Something like

MultiwordInteger::to_string() const;
FixedPoint::to_string(unsigned precision);

should work fine, possibly with some optional enum parameter for the FixedPoint version that lets you select scientific/engineering/etc notations, like the %e, %f, %g specifiers for printf, if you don't think that it's too much work. Maybe do it in a second pull request :)

I haven't thought about the implementation details too much, but I'd use a (for MultiwordInteger) char[size/3+2] as buffer and create and return an std::string from that.

For FixedPoint numbers, the needed string length should not be larger than integralWidth/3+fractionalWidth+2.

I'd put the code in the *Utility.h files. Don't worry too much about the style, I'm probably inconsistent myself and if I don't like it I'll just change stuff a bit before I merge the PR.

Please do let me know if anything's unclear :)

[Cat-Ion]

... The entries in MultiwordInteger::v are little-endian, i.e. with uint8_t as backing storage type 256 would be stored as [0, 1].