intexp: frexp for integers

Similarly to the libc frexp function, this decomposes a given integer value x into an integer coefficient c and an integral power of two n such that
c * 2n = x

ldexp is also provided as an inverse function for convenience.

$3

#### intexp(integer)
Returns [coefficient, exponent]
e.g.
``javascript
const {intexp} = require('intexp');

const decomposed = intexp(4864) // [19, 8]
`

#### ldexp(coefficient, exponent)
Returns integer given coefficient and exponent.
e.g.
`javascript
const {ldexp} = require('intexp');

const int = ldexp(19, 8) // 4864
``