minor optimisations to DataTypeMapper

[mcshaz]

as per comments in issue #36.

[rianjs]

Viewing on mobile but looks plausible. I think going forward, I'd like a unit test demonstrating that caching dictionaries are thread safe in concurrency scenarios. See the bug I fixed 2 days ago for an example test case. Basically create a unit test with .AsParallel call. If the unit test I wrote the other day covers this by invoking that machinery under the hood, that should be ok. (Just let me know that that is the case.)

Edit, pr link: #41

[mcshaz]

Apologies if I am mistaken - I would be lucky to have a tiny fraction of your programming knowledge (and I am not being humble - my day job has very little to do with computers) - I just wanted to make sure you had seen the changes did not introduce the use of dictionaries (which were pre-existing in the code) - the changes were merely:

• replacing the parameter-less constructor for a dictionary with a constructor specifying a StringComparer.OrdinalIgnoreCase, in order to prevent every lookup value for the dictionary being converted to uppercase first.
• replacing the pattern of if (dictionary.Contains(key)) {return dictionary[key]} with the dictionary.TryGetValue method - avoiding 2 lookups where 1 would do.

[rianjs]

Quite right. Much easier to see on a real computer. :)