Rename ref.is_null to ref.eq_null about reference-types

[rossberg]

I did not use "eq" in this case to avoid confusion over the fact that this instruction can be used even on types that are not eqref. However, regarding consistent naming conventions, it should probably be named ref.isnull.

[lars-t-hansen]

I don't understand why "isnull" is more consistent naming, there is equally good precedent for "is_null", considering "call_indirect", "get_local", and others.

[rossberg]

@lars-t-hansen, it's a minor thing, but I was comparing it to similar ones like eqz. Plus, so far, we have mainly been using underscore to group parts of the mnemonic that represent some kind of concept that is reappearing in a whole group of operators. Not a very strong reason, though.