C++ Try to optimize Create<Table> builders

[dbaileychess]

The generated Create<Table> methods are just collection of individual API calls that aren't particularly optimized. There must be some things we can avoid doing over and over again (e.g., reserve space once) that would speed up encoding.

Example: https://github.com/google/flatbuffers/blob/master/tests/monster_test_generated.h#L1888-L1992