Convert all numeric RDF literal (except xsd:decimals) to numbers

... all numbers are converted to xsd:doubles as proposed by @sandhawke. I choose to not convert xsd:decimals as their primary use is for values where rounding is not acceptable (think, monetary values).

I thought I just go ahead and make those changes since I had some time now and LC2 should be published tomorrow. Feel free to revert the changes if you don't agree with them. The Data Round Tripping section in the API spec includes enough warnings and is clear enough to warn implementors/users of what's gonna happen. It is referenced by all the relevant algorithms.

I will update the LC2 draft in a minute and ensure it passes validation.

/cc @gkellogg @msporny @dlongley