make wiggle-generate ordinary lib, and wiggle the proc-macro lib

this allows us to reuse the code in wiggle-generate elsewhere, because
a proc-macro=true lib can only export a #[proc_macro] and not ordinary
functions.

In lucet, I will depend on wiggle-generate to define a proc macro that
glues wiggle to the specifics of the runtime.

Cargo.toml

@@ -4,11 +4,15 @@ version = "0.1.0"
 authors = ["Pat Hickey <phickey@fastly.com>", "Jakub Konka <kubkonk@jakubkonka.com>"]
 edition = "2018"
 
+[lib]
+proc-macro = true
+
 [dependencies]
 wiggle-generate = { path = "crates/generate" }
-wiggle-runtime = { path = "crates/runtime" }
+syn = { version = "1.0", features = ["full"] }
 
 [dev-dependencies]
+wiggle-runtime = { path = "crates/runtime" }
 wiggle-test = { path = "crates/test" }
 proptest = "0.9"