Remove ScopedCredentialParameters tuple (by un-pairing type & algorithm) PR #143

All,

In PR #142 I had proposed to remove the ScopedCredentialParameters tuple
used in the makeCredential() call, replacing it instead with lists of
supported credential types and algorithms. That proposed change is now in
PR #143:

https://github.com/w3c/webauthn/pull/143

This change would remove an object, and keep RPs from having to construct
(potentially) long lists of credential type/algorithm combinations.
However, Vijay pointed out that losing this flexibility could be an issue
for RPs, as "if a new type gets defined next year, and a new algorithm the
year after that, the RP would have to backport the new algorithm to old
types as well?" [1]

As discussed on the call for 13 July 2016, whether or not this
simplification is good is dependent on how RPs implement credentials types
and algorithms. Would it be a true statement for most RP implementations
that, if a signing algorithm is supported, it would be supported for all
supported credential types? Or is the added flexibility of
picking-and-choosing necessary?

I can only make assumptions, having not designed/built an RP.

Please respond on this thread for this question; that'll inform whether we
should continue reviewing PR #143, or abandon it.

Thanks!
J.C.

1) https://github.com/w3c/webauthn/pull/142#discussion_r70354506