Add MCPI Opcode

[vlopes11]

Implement https://github.com/FuelLabs/fuel-specs/blob/master/specs/vm/opcodes.md#mcpi-memory-copy-immediate

Steps

1. Include an opcode identifier (from and to u8) in https://github.com/FuelLabs/fuel-asm/blob/master/src/opcode/consts.rs
2. Add the opcode variant with the documentation in https://github.com/FuelLabs/fuel-asm/blob/master/src/opcode.rs
   1. The constant representation defined in the previous step must be implemented in https://github.com/FuelLabs/fuel-asm/blob/master/src/opcode.rs#L1265
   2. The references to ra and rb must be defined in https://github.com/FuelLabs/fuel-asm/blob/master/src/opcode.rs#L1369
   3. The opcode into half word implementation must be added in https://github.com/FuelLabs/fuel-asm/blob/master/src/opcode.rs#L1492
3. Add the new opcode to the test cases for encode/decode: https://github.com/FuelLabs/fuel-asm/blob/master/tests/encoding.rs#L11

[sezna]

Whenever this is done, I have a branch ready for it at sezna/mcpi-opcode in sway.