Improve toNumber() behavior

[JohnBrinkman]

Currently if toNumber() cannot convert a string to a number, it returns 0. But this doesn't allow the author to know if a conversion failed -- or if the string really was a zero. Instead, we should:

• have a toNumber() failure return null
• change the coercion rules so that null to number returns zero. That would also mean the toNumber(null()) will return 0.

[JohnBrinkman]

Note that with these changes:

• toNumber("nn") returns null, but toNumber(toNumber("nn")) returns zero
• Attempting to convert bad strings to number gives an error, whereas attempting to convert arrays or objects to number results in error
• the slice operator currently interprets a null parameter as if it were omitted. e.g. [1,2,3][0:null():1] is the same as [1,2,3][0::1]. However ... the spec does not describe this behavior, and in fact says that slice parameters are coerced to number