Use ToNumber for lane arguments

[littledan]

TC39 objected to the way that functions like extractLane check that their argument is an int, rather than doing an implicit coercion. Since these are only fast when a literal int is given, I don't think this should have runtime overhead. Change the spec, polyfill and implementations to call ToNumber on lane arguments across the spec (or if anyone has counterarguments, if you say them here now I can raise them to the committee today).

[littledan]

We discussed this in the call today, and agreed to make this change. The spec text (v0.7.3) is already changed; implementations and tests need to be updated.