Implement RFC 11: Redesigning Wasmtime's APIs (#2897)

Implement Wasmtime's new API as designed by RFC 11. This is quite a large commit which has had lots of discussion externally, so for more information it's best to read the RFC thread and the PR thread.

crates/wiggle/test-helpers/src/lib.rs

@@ -1,9 +1,7 @@
 use proptest::prelude::*;
 use std::cell::UnsafeCell;
 use std::marker;
-use wiggle::{BorrowHandle, GuestMemory, Region};
-
-use wiggle_borrow::BorrowChecker;
+use wiggle::{borrow::BorrowChecker, BorrowHandle, GuestMemory, Region};
 
 #[derive(Debug, Clone)]
 pub struct MemAreas(Vec<MemArea>);
@@ -49,6 +47,9 @@ struct HostBuffer {
     cell: UnsafeCell<[u8; 4096]>,
 }
 
+unsafe impl Send for HostBuffer {}
+unsafe impl Sync for HostBuffer {}
+
 pub struct HostMemory {
     buffer: HostBuffer,
     bc: BorrowChecker,