Should APFloat's construct-from-string take a string length?

I noticed that APInt’s construct-from-string takes the length of the string, but APFloat does not. Would it make sense to modify APFloat to also take a length? I would imagine that it’d help protect us against malformed string values.

This sounds reasonable to me. Note that you can change them both to
take a const StringRef& and get the standard variants (std::string,
const char*, ptr + size) for free.

- Daniel