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`.

Cargo.lock

@@ -6334,8 +6334,6 @@ dependencies = [
  "group",
  "k256",
  "rand_core 0.6.4",
- "rustversion",
- "std-shims",
  "zeroize",
 ]