Assembler/CodeBuffer::emit_int* accept signed int arguments.

- Since we are trying to emit some bit patterns into the code buffer instead of doing integer arithmetics, it makes more sense to use unsigned parameters.

- It makes usage with constants become inconvenient, as an integer literal is positive, a 0xC0 does not fit into an int8_t, resulting in the compiler complaining about lossy implicit conversions, the current solution is manual casting of the constants to unsigned char, which can be converted to int8_t without complaints.