wiggle-generate: paramaterize library on module path to runtime (#1574)

* wiggle-generate: paramaterize library on module path to runtime

This change makes no functional difference to users who only use the
wiggle crate.

Add a parameter to the `Names` constructor that determines the module
that runtime components (e.g. GuestPtr, GuestError etc) of wiggle come
from. For `wiggle` users this is just `quote!(wiggle)`, but other
libraries which consume wiggle-generate may wrap and re-export wiggle
under some other path, and not want their consumers to have to know
about the wiggle dependency, e.g. `quote!(my_crate::some_path::wiggle)`.

* wiggle-generate,macro: move more logic into macro

better for code reuse elsewhere

crates/wiggle/generate/src/config.rs

@@ -12,7 +12,6 @@ use syn::{
 pub struct Config {
     pub witx: WitxConf,
     pub ctx: CtxConf,
-    pub emit_metadata: bool,
 }
 
 #[derive(Debug, Clone)]
@@ -60,7 +59,6 @@ impl Config {
             ctx: ctx
                 .take()
                 .ok_or_else(|| Error::new(err_loc, "`ctx` field required"))?,
-            emit_metadata: false,
         })
     }
 }