Optimize some functions in the wiggle crate (#5566)

* wiggle: Inline some trivial functions

This commit marks a number of functions in wiggle as `#[inline]` as
they're otherwise trivial, mostly returning constants. This comes out of
some work I looked at recently with Andrew where some of these functions
showed up in profiles when they shouldn't.

* wiggle: Optimize the `GuestMemory` for shared memory

This commit implements a minor optimization to the `GuestMemory`
implementation for Wasmtime to skip most methods if a shared memory is
in play. Shared memories never get borrowed and this can be used to
internally skip some borrow-checker methods.

* wiggle: Optimize `GuestPtr::to_vec`

This commit replaces the safe implementation of `GuestPtr::to_vec` with
an unsafe implementation. The purpose of this is to speed up the
function when used with shared memory which otherwise performs a bunch
of atomic reads for types like `u8` which does validation-per-element
and isn't vectorizable. On a benchmark I was helping Andrew with this
sped up the host code enough to the point that guest code dwarfed the
execution time.

* Fix build

crates/wiggle/src/lib.rs

@@ -610,10 +610,22 @@ impl<'a, T> GuestPtr<'a, [T]> {
         T: GuestTypeTransparent<'a> + Copy + 'a,
     {
         let guest_slice = self.as_unsafe_slice_mut()?;
-        let mut vec = Vec::with_capacity(guest_slice.ptr.len());
-        for offs in 0..guest_slice.ptr.len() {
-            let elem = self.get(offs as u32).expect("already validated the size");
-            vec.push(elem.read()?);
+        let len = guest_slice.ptr.len();
+        let mut vec = Vec::with_capacity(len);
+
+        // SAFETY: The `guest_slice` variable is already a valid pointer into
+        // the guest's memory, and it may or may not be a pointer into shared
+        // memory. We can't naively use `.to_vec(..)` which could introduce data
+        // races but all that needs to happen is to copy data into our local
+        // `vec` as all the data is `Copy` and transparent anyway. For this
+        // purpose the `ptr::copy` function should be sufficient for copying
+        // over all the data.
+        //
+        // TODO: audit that this use of `std::ptr::copy` is safe with shared
+        // memory (https://github.com/bytecodealliance/wasmtime/issues/4203)
+        unsafe {
+            std::ptr::copy(guest_slice.ptr.as_ptr().cast::<T>(), vec.as_mut_ptr(), len);
+            vec.set_len(len);
         }
         Ok(vec)
     }