Implementation default GetHashCode for value types

[tgiphil]

Here's the plan:

• For value types without references fields or empty space between fields, return XOR all the bytes of the structure.
• For all other value types, return the first non-reference field.

Implementation:

Emit the starting location and size of the value type data for the XOR computation within the runtime tables. Then the runtime would calculate the hash code given the information in the runtime tables.