Feat/add new functions

[aritkulova]

Added:

• logical functions (not, or, and);
• safe functions for add, subtract, multiply, divide for u8, u16, u32, u64.

Removed:

• unsuported "pub" modifier.

Refactored:

• added enum in assets file for better testing experience.

[apoelstra]

Can you please break this into a reviewable series of PRs?

[aritkulova]

@apoelstra Yeah, will do