Place zcash_proofs::prover::LocalTxProver behind a feature flag

This enables zcash_proofs to be compiled to WASM, which the directories
crate doesn't support.

zcash_proofs/src/lib.rs

@@ -1,13 +1,15 @@
 extern crate bellman;
 extern crate blake2b_simd;
 extern crate byteorder;
-extern crate directories;
 extern crate ff;
 extern crate pairing;
 extern crate rand_os;
 extern crate sapling_crypto;
 extern crate zcash_primitives;
 
+#[cfg(feature = "local-prover")]
+extern crate directories;
+
 use bellman::groth16::{prepare_verifying_key, Parameters, PreparedVerifyingKey, VerifyingKey};
 use pairing::bls12_381::Bls12;
 use std::fs::File;
@@ -15,9 +17,11 @@ use std::io::{self, BufReader};
 use std::path::Path;
 
 mod hashreader;
-pub mod prover;
 pub mod sapling;
 
+#[cfg(feature = "local-prover")]
+pub mod prover;
+
 pub fn load_parameters(
     spend_path: &Path,
     spend_hash: &str,