Adds typedefs and functions to help write generic code.

Adds typedefs and functions to help write generic code.

The following changes have been made to the C++ codegen to enable writing generic code
that uses the Table and NativeTable types.

- Adds TableType and NativeTableType typedefs to NativeTable and Table structs.
- Adds GetFullyQualifiedName() to NativeTables if --gen-name-strings is set.
- Adds a static Pack function to Tables that simply calls the global CreateX functions.

See cr/140391505 as an example of improved usage.

From cl/140529288.

Change-Id: Idec137c16129e15c1783f94fabdcea24aeeaaef6