feat: add fixed-capacity, statically-allocated type tflite::StaticVector

[rkuester]

Add a type, tflite::StaticVector, which behaves like std::vector, but which avoids heap memory allocation.

BUG=#2636

[rkuester]

To my knowledge, we don't have any type like this yet—something that behaves like a std::vector, but can be statically allocated. Am I wrong?

This is used in upcoming PRs.

[rkuester]

These changes required #2648.