Add instruction encoding for "C" extension

[michaelmelanson]

When implementing the "C" extension set, I didn't implement instruction encoding like I did for the other instruction formats.

Be sure to add all the appropriate test cases, using encoding_test to ensure that decoding some bytes then re-encoding them again produces the original bytes. Here's an example test:

riscy/isa/src/lib.rs

Lines 155 to 162 in 2cee748

	encoding_test(
	&[183, 2, 1, 0],
	Instruction::U {
	opcode: Opcode::Lui,
	imm: 0x10000,
	rd: Register::T0,
	},
	);

Instruction formats

These instruction formats are documented in Chapter 12 ("C” Standard Extension for Compressed Instructions, Version 2.0) of the RISC-V specification:

• CIW
• CS
• CL
• CI
• CNOP / CB
• CA
• CJ
• CR
• CSS