Don't copy VMBuiltinFunctionsArray into each VMContext (#3741)

* Don't copy `VMBuiltinFunctionsArray` into each `VMContext`

This is another PR along the lines of "let's squeeze all possible
performance we can out of instantiation". Before this PR we would copy,
by value, the contents of `VMBuiltinFunctionsArray` into each
`VMContext` allocated. This array of function pointers is modestly-sized
but growing over time as we add various intrinsics. Additionally it's
the exact same for all `VMContext` allocations.

This PR attempts to speed up instantiation slightly by instead storing
an indirection to the function array. This means that calling a builtin
intrinsic is a tad bit slower since it requires two loads instead of one
(one to get the base pointer, another to get the actual address).
Otherwise though `VMContext` initialization is now simply setting one
pointer instead of doing a `memcpy` from one location to another.

With some macro-magic this commit also replaces the previous
implementation with one that's more `const`-friendly which also gets us
compile-time type-checks of libcalls as well as compile-time
verification that all libcalls are defined.

Overall, as with #3739, the win is very modest here. Locally I measured
a speedup from 1.9us to 1.7us taken to instantiate an empty module with
one function. While small at these scales it's still a 10% improvement!

* Review comments

crates/runtime/src/instance/allocator.rs

@@ -481,10 +481,8 @@ unsafe fn initialize_vmcontext(instance: &mut Instance, req: InstanceAllocationR
     }
 
     // Initialize the built-in functions
-    ptr::write(
-        instance.vmctx_plus_offset(instance.offsets.vmctx_builtin_functions_begin()),
-        VMBuiltinFunctionsArray::initialized(),
-    );
+    *instance.vmctx_plus_offset(instance.offsets.vmctx_builtin_functions()) =
+        &VMBuiltinFunctionsArray::INIT;
 
     // Initialize the imports
     debug_assert_eq!(req.imports.functions.len(), module.num_imported_funcs);