Hstore query support

[yinzara]

This PR adds support for querying on hstore type columns which can map to either Dictionary<string, string> or ImmutableDictionary<string, string> in C#.

The following methods and properties are available for translation:

• Dictionary<string, string>.Item (i.e. myDictionary["key"])
• Dictionary<string, string>.Item
• Dictionary<string, string>.ContainsKey
• ImmutableDictionary<string, string>.ContainsKey
• Dictionary<string, string>.ContainsValue
• ImmutableDictionary<string, string>.ContainsValue
• Dictionary<string, string>.Count
• ImmutableDictionary<string, string>.Count
• ImmutableDictionary<string, string>.IsEmpty
• Enumerable.Any

AFAIK this is all the properties that could ever support translation except those that modify the hstore.
I attempted Keys and Values and could never get it to work correctly.

Fixes #212

[yinzara]

Comparing this line to line 186, you see that when using a normal Dictionary<string, string>, accessing the indexer property causes the key being passed as the index to be a SqlConstantExpression while if use use an ImmutableDictionary<string, string>, the key being passed as the index is instead a SqlParameterExpression.

With the Nullable dictionary examples, you see the same behavior however you can see that for the ContainsKey translation, the key is being passed as the SqlParameterExpression both in the normal Dictionary and the ImmutableDictionary.

I have wracked my brain and looked throughout the codebase on how this could possibly happen. I do not treat the two types of dictionaries any differently in the hstore code I've added so the variation seems extremely perplexing to me.

This technically will not cause a functional issue. It will work either way.

However, because the Dictionary<string, string> access is being passed as a constant and not a parameter, if a 2nd query is made on a different key in the same running application, it will require a completely new query plan and not reuse the existing cached query.

Anyone have any ideas why this might be happening?

I don't think this should really hold up the PR, it's just strange :-)