Classic BPF instruction definitions and encoding.

BPF has a simple instruction set with:

• A: 32-bit accumulator
• X: 32-bit index register
• M[0-15]: 16 x 32-bit scratch memory slots

Each instruction is represented as a tuple. Examples:

• {:ld, :b, [:k, 0]} - load byte at offset 0 into A
• {:add, 4} - add 4 to A
• {:jmp, :jeq, :k, 5, 1, 0} - if A == 5, skip 1, else skip 0
• :txa - copy X to A
• {:ret, :k, 0xFFFFFFFF} - return constant