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/macro/Cargo.toml

@@ -16,6 +16,7 @@ proc-macro = true
 [dependencies]
 wiggle-generate = { path = "../generate", version = "0.15.0" }
 witx = { path = "../../wasi-common/WASI/tools/witx", version = "0.8.5" }
+quote = "1.0"
 syn = { version = "1.0", features = ["full"] }
 
 [dev-dependencies]