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/batch.rs

@@ -1,4 +1,4 @@
-use std_shims::vec::Vec;
+use alloc::vec::Vec;
 
 use rand_core::{RngCore, CryptoRng};