Make ToNumber(String) conversion more spec-compliant

[sainaen]

Update ToNumber conversion applied to the String type:

• plus (+) or minus (-) signs are not allowed before the hex literals,
  return NaN for them
• also return NaN for HexIntegralLiteral followed by anything other than
  whitespace (in contrast to how 'parseInt()' works)
• add support for binary ('0b101010') and octal ('0o52') literals

New behavior is applied only if the language version is >=ES6, so it
shouldn’t brake any existing code.

This also enables built-ins/Number test262 cases.

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Fixes #368.

I’ve also verified that the new ToNumberLegacyConversionsTest works in the master branch without this patch, so it in fact represents the ToNumber's current behavior.

[sainaen]

I have one question related to this.

I decided to not change the tests that cover some non-spec’ed functionality in MozSuite, which leaves it on the language version < ES6. Is it ok?
I think in SpiderMonkey they slowly remove parts of their own tests that are covered by test262, maybe that's something we should consider too?

[gbrail]

Looks good to me -- I understand what you did to this complex code and I see that it's tested and works for my simple tests as well.

Curious if anyone else has also inspected this.

I do think it's safest to leave this to ">=ES6" for now since backward compatibility can show up at the worst time!

[sainaen]

Thanks for merging!

I do think it's safest to leave this to ">=ES6" for now since backward compatibility can show up at the worst time!

Sure, let’s leave it for now.

It’s just that I’m a bit concerned it may become harder to update in future if we drift away too much. Maybe create an issue to keep track of it?