Uint conversion functions

[Hrom131]

Describe the feature

Simplicityhl-std must include functions for convenient uint type conversion.

Functions for safe conversion:

1. From u8 to: u16, u32, u64, u128, u256
2. From u16 to: u32, u64, u128, u256
3. From u32 to: u64, u128, u256
4. From u64 to: u128, u256
5. From u128 to: u256

Functions for conversion with overflow checking:

1. From u16 to: u8
2. From u32 to: u16, u8
3. From u64 to: u32, u16, u8
4. From u128 to: u64, u32, u16, u8
5. From u256 to: 128, u64, u32, u16, u8