Create benchmarks on CPU and memory

[angularsen]

To create a baseline when discussing moving from struct to class (#372,#500) or changing the internal representation from double to decimal (#372) or even to a rational value (#478).

Any changes should be compared to a baseline so we know how much better/worse things get in terms of CPU and memory and in both absolute and relative terms.

1. CPU time
2. Garbage collector pressure
3. Memory footprint
   • Single quantity instances
   • Array of quantity instances

Example of things to test on:

• Construction: constructors and From methods
• Conversion: .As(), .ToUnit() and conversion properties like .Centimeters
• ToString: and its various overloads
• Parsing: Parse and TryParse methods
• Dynamic: The above, but dynamically via IQuantity and QuantityInfo types. Should introduce additional boxing and extra conditions.

I suggest using Benchmark.NET, I have good experience with it.

[tmilnthorp]

Benchmark.NET looks pretty awesome. I have no experience with it, but looks like it will do what we need it to.

[angularsen]

Fixed in #637. Can be improved more later.