Function signextend 

pub fn signextend<WIRE, H>(interpreter: &mut Interpreter<WIRE>, _host: &mut H)
where
    WIRE: InterpreterTypes,
    H: Host + ?Sized,

Implements the SIGNEXTEND opcode as defined in the Ethereum Yellow Paper.

In the yellow paper SIGNEXTEND is defined to take two inputs, we will call them x and y, and produce one output.

The first t bits of the output (numbering from the left, starting from 0) are equal to the t-th bit of y, where t is equal to 256 - 8(x + 1).

The remaining bits of the output are equal to the corresponding bits of y.

Note: If x >= 32 then the output is equal to y since t <= 0.

To efficiently implement this algorithm in the case x < 32 we do the following.

Let b be equal to the t-th bit of y and let s = 255 - t = 8x + 7 (this is effectively the same index as t, but numbering the bits from the right instead of the left).

We can create a bit mask which is all zeros up to and including the t-th bit, and all ones afterwards by computing the quantity 2^s - 1.

We can use this mask to compute the output depending on the value of b.

If b == 1 then the yellow paper says the output should be all ones up to and including the t-th bit, followed by the remaining bits of y; this is equal to y | !mask where | is the bitwise OR and ! is bitwise negation.

Similarly, if b == 0 then the yellow paper says the output should start with all zeros, then end with bits from b; this is equal to y & mask where & is bitwise AND.