show opcode as 7-bits like all other insn for unzip and zip

[psherman42]

[ 00100 ][ 11 ] should be written as [ 0010011 ] (0x13) or OP-IMM to be consistent with all other instructions.

Also, in the Operations section, should one of the foreach loops between unzip and zip be different like, for example,

foreach(i from 0 to xlen/2-1) {
    x(rd)[i] = X(rs1)[2*i]
    x(rd)[2*i+1] = X(rs1)[i+xlen/1]
}