higher-level optimizations such as NN_OPTIMIZE_RAM.

[GiorgosXou]

Since optimizations get increased through time, I thought it would be a great idea to have higher-level combo\group optimization names for them too, like:

#define NN_OPTIMIZE_RAM
#define NN_OPTIMIZE_CPU
#define NN_OPTIMIZE_FLASH

or even better add versions, too... so there won't be breaking changes, eg.

#define NN_OPTIMIZE_RAM_V1
...