Function: fromdigits
Section: conversions
C-Name: fromdigits
Prototype: GDG
Help: fromdigits(x,{b=10}): gives the integer formed by the elements of x seen
 as the digits of a number in base b.
Doc: gives the integer formed by the elements of $x$ seen as the digits of a
 number in base $b$ ($b = 10$ by default); $b$ must be an integer satisfying
 $|b|>1$. This is the reverse of \kbd{digits}:
 \bprog
 ? digits(1234, 5)
 %1 = [1,4,4,1,4]
 ? fromdigits([1,4,4,1,4],5)
 %2 = 1234
 @eprog\noindent By convention, $0$ has no digits:
 \bprog
 ? fromdigits([])
 %3 = 0
 @eprog\noindent This function works with $x$ a \typ{VECSMALL}; and
 also with $b < 0$ or $x[i]$ not an actual digit in base $b$ (i.e.,
 $x[i] < 0$ or $x[i] \geq b$): if $x$ has length $n$, we return
 $\sum_{i=1}^{n} x[i] b^{n-i}$.
