Tweak multiexp to compile on core

On `core`, it'll use a serial implementation of no benefit other than the fact
that when `alloc` _is_ enabled, it'll use the multi-scalar multiplication
algorithms.

`schnorr-signatures` was prior tweaked to include a shim for
`SchnorrSignature::verify` which didn't use `multiexp_vartime` yet this same
premise. Now, instead of callers writing these shims, it's within `multiexp`.

crypto/multiexp/src/pippenger.rs

@@ -1,3 +1,5 @@
+use alloc::vec;
+
 use zeroize::Zeroize;
 
 use ff::PrimeFieldBits;