Rewrite exp(n*ln(x)) as a power : exp2pow

### 2.23.8 Rewrite exp(n*ln(x)) as a power : `exp2pow`

`exp2pow` rewrites expression of the form exp(n*ln(x)) as a power of x.
Input :

`exp2pow(exp(n*ln(x)))`

Output :

`x```^```n`

Note the difference with `lncollect` :
`lncollect(exp(n*ln(x))) = exp(n*log(x))`
`lncollect(exp(2*ln(x))) = exp(2*log(x))`
`exp2pow(exp(2*ln(x))) = x```^```2`
But :
`lncollect(exp(ln(x)+ln(x))) = x```^```2`
`exp2pow(exp(ln(x)+ln(x))) = x```^```(1+1)`