Modify IReadableStringCollection to allow better querying of values

[pranavkm]

• Add a Keys and Count method so it looks somewhat like a collection
• Modify GetValue to return a struct that returns either a single value or a list of values. This would avoid extra array allocations for the most common case when keys have exactly one value.

[loudej]

The strict would only need to be in the enumeration, getvalue/getvalues are simplest if they return string string[]

[davidfowl]

@pranavkm @loudej This was fixed in that PR right?

[pranavkm]

It wasn't.

[Tratcher]

Count, Keys, Values, ContainsKey