Add procedural unsafe_gc_impl! macro for manual impls (#22)

All `manually_traced` types use this new macro, making the implementation more concise.

Procedural macros are more flexible than what we use currently and
single macro for all our manually traced code.
This is now a (semi-public) API that downstream crates can start to utilize.

Unfortunately, this increases the compile time and dependency tree.
The core crate now depends on zerogc-derive and syn (`features=["full"]`).


* [derive] Use `crate::` when deriving inside zerogc

This is the proc-macro equivelant of '$crate'
This allows us to use derive/proc-macros inside of zerogc itself

Author: Techcable

Cargo.toml

@@ -13,6 +13,8 @@ readme = "README.md"
 # Providing support for these important libraries,
 # gives zerogc 'batteries included' support.
 indexmap = { version = "1.6", optional = true }
+# Used for macros
+zerogc-derive = { path = "libs/derive", version = "0.2.0-alpha.1" }
 
 [workspace]
 members = ["libs/simple", "libs/derive", "libs/context"]

libs/derive/Cargo.toml

@@ -16,6 +16,6 @@ zerogc = { version = "0.2.0-alpha.1", path = "../.." }
 
 [dependencies]
 # Proc macros
-syn = "1.0.55"
+syn = { version = "1.0.55", features = ["full", "extra-traits"] }
 quote = "1.0.8"
 proc-macro2 = "1"