Refactor cmake build flags
Add arm linux toolchains
Add macOS+iOS build support
Support windows
Add library install
Refactor gitlab CI

1. Fix random device chosen bug.
2. Add random device chosen strategy for running model.

1. Abstact android and arm linux to one format
2. Support cross compilation for ARM linux
3. Related issue #36

1. Move all files in kernels to ops
2. unify the op_def_registry and op_registry.
3. Support op clip: do not link the ops' code when the op is not registered.