Disable sapling-crypto default features by default

The only default-enabled feature flag in `sapling-crypto` is the
`multicore` feature flag, which we re-export in each crate that includes
proof creation. We need to disable it as a default feature of our
dependency in order to enable it to be correctly disabled when a user of
e.g. `zcash_primitives` disables its default features.

Cargo.toml

@@ -54,7 +54,7 @@ bitvec = "1"
 blake2s_simd = "1"
 bls12_381 = "0.8"
 jubjub = "0.10"
-sapling = { package = "sapling-crypto", version = "0.1.3" }
+sapling = { package = "sapling-crypto", version = "0.1.3", default-features = false }
 
 # - Orchard
 nonempty = "0.7"