Revamp object system, start using gc crate

Wow, what a ride. I think everything should be working now. In short:

* Objects use the `gc` crate, which as a `Gc` garbage-collected pointer
  type. I may choose to implement my own in contiguous memory in the
  future. We will see.
* The type system is no longer global. This is a bit of a burden,
  because now, whenever you want to create a new object, you need to
  pass its type object into the `Obj::instantiate` method, as well as
  its `::create` static method.
* This burden is somewhat alleviated by the `ObjFactory` trait, which
  helps create new objects as long as you have access to a `builtins`
  hashmap. So something that would normally look like this:

    fn init_builtins(builtins: &mut HashMap<String, ObjP>) {
        let print_builtin = upcast_obj(BuiltinFunctionInst::create(
            ObjP::clone(&builtins.get("BuiltinFunction").unwrap()),
            "print",
            print,
            1
        );
        builtins.insert("print".to_string(), print_builtin)
        // other builtins inserted here...
    }

  now looks like this:

    fn init_builtins(builtins: &mut HashMap<String, ObjP>) {
        let print_builtin = builtins.create_builtin_function("print", print, 1);
        builtins.insert("print".to_string(), print_builtin);
    }

(turns out, if all you need is a HashMap<String, ObjP>, you can
implement ObjFactory for HashMap<String, ObjP> itself(!))

Overall, I'm happier with this design, and I think this is what is going
to get merged. It's a little weird to be querying type names that are
used in the language itself to get those type objects, but whatever
works, I guess.

Next up is vtables.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

Cargo.lock

@@ -88,7 +88,7 @@ dependencies = [
  "heck",
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.77",
 ]
 
 [[package]]
@@ -109,6 +109,27 @@ version = "0.1.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f3f6d59c71e7dc3af60f0af9db32364d96a16e9310f3f5db2b55ed642162dd35"
 
+[[package]]
+name = "gc"
+version = "0.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f73a03797d58caede765f4b19522d1a63d737088bbef81de06d2dd117d7bc8c3"
+dependencies = [
+ "gc_derive",
+]
+
+[[package]]
+name = "gc_derive"
+version = "0.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fb6f1e0d69658b2806f60864bad1a80a9634fc0bc17f7ed55828e3b9c45bf61e"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 1.0.109",
+ "synstructure",
+]
+
 [[package]]
 name = "heck"
 version = "0.5.0"
@@ -128,6 +149,7 @@ dependencies = [
  "assert_matches",
  "clap",
  "common_macros",
+ "gc",
  "thiserror",
 ]
 
@@ -155,6 +177,17 @@ version = "0.11.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7da8b5736845d9f2fcb837ea5d9e2628564b3b043a70948a3f0b778838c5fb4f"
 
+[[package]]
+name = "syn"
+version = "1.0.109"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "72b64191b275b66ffe2469e8af2c1cfe3bafa67b529ead792a6d0160888b4237"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "unicode-ident",
+]
+
 [[package]]
 name = "syn"
 version = "2.0.77"
@@ -166,6 +199,18 @@ dependencies = [
  "unicode-ident",
 ]
 
+[[package]]
+name = "synstructure"
+version = "0.12.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f36bdaa60a83aca3921b5259d5400cbf5e90fc51931376a9bd4a0eb79aa7210f"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 1.0.109",
+ "unicode-xid",
+]
+
 [[package]]
 name = "thiserror"
 version = "1.0.63"
@@ -183,7 +228,7 @@ checksum = "a4558b58466b9ad7ca0f102865eccc95938dca1a74a856f2b57b6629050da261"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn",
+ "syn 2.0.77",
 ]
 
 [[package]]
@@ -192,6 +237,12 @@ version = "1.0.13"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e91b56cd4cadaeb79bbf1a5645f6b4f8dc5bde8834ad5894a8db35fda9efa1fe"
 
+[[package]]
+name = "unicode-xid"
+version = "0.2.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ebc1c04c71510c7f702b52b7c350734c9ff1295c464a03335b00bb84fc54f853"
+
 [[package]]
 name = "utf8parse"
 version = "0.2.2"