Pin future-core version to 0.3.29

Workaround https://github.com/rust-lang/rust/issues/32976 from 0.3.30

Change-Id: I14937c0e2139bd335ad1e3ea061c0c8160df28b4

Cargo.toml

diff --git a/Cargo.toml b/Cargo.toml
index ac8f6df..3979b49 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -19,6 +19,7 @@
 libtock_core = { path = "core" }
 libtock_codegen = { path = "codegen" }
 futures = { version = "0.3.1", default-features = false, features = ["unstable", "cfg-target-has-atomic"] }
+futures-core = { version = "=0.3.29", default-features = false, features = ["unstable", "cfg-target-has-atomic"] }
 
 [dev-dependencies]
 corepack = { version = "0.4.0", default-features = false, features = ["alloc"] }