Fix the build by updating `linked_list_allocator` to 0.8.7.

`linked_list_allocator` requires you to specify the `const_mut_refs` option in order for `Heap::empty` to be `const`. This broke the build of `libtock_core` when `linked_list_allocator` 0.8.7 was pushed to crates.io.

core/Cargo.toml

diff --git a/core/Cargo.toml b/core/Cargo.toml
index 5632d23..c6dec5e 100644
--- a/core/Cargo.toml
+++ b/core/Cargo.toml
@@ -10,7 +10,12 @@
 custom_panic_handler = []
 custom_alloc_error_handler = []
 
+[dependencies.linked_list_allocator]
+default-features = false
+features = ["const_mut_refs"]
+optional = true
+version = "0.8.7"
+
 [dependencies]
-linked_list_allocator = { optional = true, version = "0.8.6", default-features = false }
 libtock_codegen = { path = "../codegen" }
 libtock_platform = { path = "platform" }