Build against grin 5.4.0 and the deps-merged contracts branch

Fixes mwixnet#32 (no longer builds on current Rust). The grin node deps move to
the 5.4.0 release crates; grin-wallet points at jwinterm/grin-wallet contracts work
pinned by revision. Drops unused grin_servers/chain/p2p from the main build
(grin_servers isn't published).

- Cargo.toml: grin_* -> 5.4.0; grin_wallet_* -> contracts fork (rev-pinned).
- secp::random_secret now takes use_test_rng; pass false at all call sites.
- create_onion / ComSignature::sign take a use_test_rng bool; pass false in tests.
- Test modules reference the file-level onion alias via super::grin_onion.
- tests/e2e.rs (boots a real node+miner) gated behind a new `integration` feature
  so a default `cargo test` runs the unit suite (18 tests) without a node.

src/node.rs

@@ -248,7 +248,7 @@ pub mod mock {
 	use grin_core::core::hash::Hash;
 	use grin_core::core::Transaction;
 
-	use grin_onion::crypto::secp::Commitment;
+	use super::grin_onion::crypto::secp::Commitment;
 
 	use super::{GrinNode, NodeError};