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.

crates/generate/Cargo.toml

@@ -5,7 +5,6 @@ authors = ["Pat Hickey <phickey@fastly.com>", "Jakub Konka <kubkon@jakubkonka.co
 edition = "2018"
 
 [lib]
-proc-macro = true
 
 [dependencies]
 wiggle-runtime = { path = "../runtime" }