Function: fingerprint
Section: programming/specific
C-Name: fingerprint
Prototype: GD64,L,
Help: fingerprint(x, {b = 64}): return a b-bit fingerprint of x.
Doc: return a $b$-bit fingerprint of $x$, assuming $b > 0$
 ($b = 64$ if omitted). Not suitable for cryptographic use
 or in an adversarial context: collisions are easily generated.
 For objects definable on both 32 and 64 bit architectures, the returned
 value is the same. The main use is to implement unit tests and check results
 against fingerprints for the expected outputs, not so much for hashtables.
Variant: Also available is \fun{ulong}{hash_GEN}{GEN x}, which is faster
 and returns an \kbd{ulong} value, i.e., 64-bit on 64-bit architectures
 and 32-bit otherwise. This latter function is ideal for internal hashtables
 but its values are architecture dependent.
