Add built-in functions

[SkymanOne]

Follow-up for #7

Introduce generic built-in functions for operations on lists and safe maths:

• map(list, func) -> List/Set<Generic<int, uint, float, address, hex, list, set>>
• map(list, func) -> List/Set<Generic<int, uint, float, address, hex, list, set>>
• add(list, item) -> List/Set<Generic<int, uint, float, address, hex, list, set>>
• remove(list, item) -> List/Set<Generic<int, uint, float, address, hex, list, set>>
• get(list, item) -> Generic<int, uint, float, address, hex, list, set>
• handle(expression, func) -> Generic<int, uint, float> - for safe math operation
• size(list) -> uint
• keys(mapping) -> Set<Generic<int, uint, float, address, hex>>
• value(mapping) -> list
• insert(mapping, key, value) -> mapping
• value(mapping, key) -> Generic<int, uint, float, address, hex, list, set>