Rewrite for recursive safety

This commit rewrites the runtime crate to provide safety in the face
of recursive calls to the guest. The basic principle is that
`GuestMemory` is now a trait which dynamically returns the
pointer/length pair. This also has an implicit contract (hence the
`unsafe` trait) that the pointer/length pair point to a valid list of
bytes in host memory "until something is reentrant".

After this changes the various suite of `Guest*` types were rewritten.
`GuestRef` and `GuestRefMut` were both removed since they cannot safely
exist. The `GuestPtrMut` type was removed for simplicity, and the final
`GuestPtr` type subsumes `GuestString` and `GuestArray`. This means
that there's only one guest pointer type, `GuestPtr<'a, T>`, where `'a`
is the borrow into host memory, basically borrowing the `GuestMemory`
trait object itself.

Some core utilities are exposed on `GuestPtr`, but they're all 100%
safe. Unsafety is now entirely contained within a few small locations:

* Implementations of the `GuestType` for primitive types (e.g. `i8`,
  `u8`, etc) use `unsafe` to read/write memory. The `unsafe` trait of
  `GuestMemory` though should prove that they're safe.

* `GuestPtr<'_, str>` has a method which validates utf-8 contents, and
  this requires `unsafe` internally to read all the bytes. This is
  guaranteed to be safe however given the contract of `GuestMemory`.

And that's it! Everything else is a bunch of safe combinators all built
up on the various utilities provided by `GuestPtr`. The general idioms
are roughly the same as before, with various tweaks here and there. A
summary of expected idioms are:

* For small values you'd `.read()` or `.write()` very quickly. You'd
  pass around the type itself.

* For strings, you'd pass `GuestPtr<'_, str>` down to the point where
  it's actually consumed. At that moment you'd either decide to copy it
  out (a safe operation) or you'd get a raw view to the string (an
  unsafe operation) and assert that you won't call back into wasm while
  you're holding that pointer.

* Arrays are similar to strings, passing around `GuestPtr<'_, [T]>`.
  Arrays also have a `iter()` method which yields an iterator of
  `GuestPtr<'_, T>` for convenience.

Overall there's still a lot of missing documentation on the runtime
crate specifically around the safety of the `GuestMemory` trait as well
as how the utilities/methods are expected to be used. Additionally
there's utilities which aren't currently implemented which would be easy
to implement. For example there's no method to copy out a string or a
slice, although that would be pretty easy to add.

In any case I'm curious to get feedback on this approach and see what
y'all think!

tests/ints.rs

@@ -1,6 +1,6 @@
 use proptest::prelude::*;
 use std::convert::TryFrom;
-use wiggle_runtime::GuestError;
+use wiggle_runtime::{GuestError, GuestMemory};
 use wiggle_test::{impl_errno, HostMemory, MemArea, WasiCtx};
 
 wiggle::from_witx!({
@@ -44,22 +44,20 @@ impl CookieCutterExercise {
     }
 
     pub fn test(&self) {
-        let mut ctx = WasiCtx::new();
-        let mut host_memory = HostMemory::new();
-        let mut guest_memory = host_memory.guest_memory();
+        let ctx = WasiCtx::new();
+        let host_memory = HostMemory::new();
 
         let res = ints::cookie_cutter(
-            &mut ctx,
-            &mut guest_memory,
+            &ctx,
+            &host_memory,
             self.cookie.into(),
             self.return_ptr_loc.ptr as i32,
         );
         assert_eq!(res, types::Errno::Ok.into(), "cookie cutter errno");
 
-        let is_cookie_start = *guest_memory
+        let is_cookie_start = host_memory
             .ptr::<types::Bool>(self.return_ptr_loc.ptr)
-            .expect("ptr to returned Bool")
-            .as_ref()
+            .read()
             .expect("deref to Bool value");
 
         assert_eq!(