DynamicParameters should implement IEnumerable to support collection initializer syntax

This is related to https://github.com/StackExchange/Dapper/issues/849 and https://github.com/StackExchange/Dapper/issues/304 but not exactly.

Please let DynamicParameters implement IEnumerable so that it supports collection initializer syntax. It is a slightly cleaner way or registering params.

Compare:

DynamicParameters dynamicParams = new DynamicParameters();
dynamicParams.Add("@sessionId", exportSession.Id);
dynamicParams.Add("@skipValidation", exportSession.SkipValidation);
dynamicParams.Add("@result", dbType: DbType.Int32, direction: ParameterDirection.Output);

vs

DynamicParameters dynamicParams = new DynamicParameters
{
    { "@sessionId", exportSession.Id },
    { "@skipValidation", exportSession.SkipValidation },
    { "@result", dbType: DbType.Int32, direction: ParameterDirection.Output },
};

Slightly less verbose.

I understand that you need to decide on the GetEnumerator part, as to what to return. I learn that there is little hesitancy on exposing Parameters as such. Maybe even iterating over ParameterNames would do. Or else iterate over <ParameterName, obj Value>. But really it should have been <ParameterName, ParameterInfo> if you were open to it.

To consider

If iterating over ParameterNames, now we have two ways to iterate over parameter names, but I guess it is OK.

Start at DynamicParameters and review related issues #849 and #304 for prior discussion. Determine the intended GetEnumerator behavior and collection-initializer compatibility, then verify that the chosen parameter representation can be enumerated consistently and that the requested syntax works.