add a lifetime to the wiggle_runtime::GuestErrorType trait (#41)

* add a lifetime to the wiggle_runtime::GuestErrorType trait, wiggle_tests::WasiCtx struct

* wiggle-generate: make config parsing public so it can be reused in lucet

tests/strings.rs

@@ -9,7 +9,7 @@ wiggle::from_witx!({
 
 impl_errno!(types::Errno);
 
-impl strings::Strings for WasiCtx {
+impl<'a> strings::Strings for WasiCtx<'a> {
     fn hello_string(&self, a_string: &GuestPtr<str>) -> Result<u32, types::Errno> {
         let mut bc = GuestBorrows::new();
         let s = a_string.as_raw(&mut bc).expect("should be valid string");