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

crates/runtime/src/guest_type.rs

@@ -1,7 +1,7 @@
 use crate::{GuestError, GuestPtr};
 use std::mem;
 
-pub trait GuestErrorType {
+pub trait GuestErrorType<'a> {
     type Context;
     fn success() -> Self;
     fn from_error(e: GuestError, ctx: &Self::Context) -> Self;