Further minor optimizations to instantiation (#3791)

* Shrink the size of `FuncData`

Before this commit on a 64-bit system the `FuncData` type had a size of
88 bytes and after this commit it has a size of 32 bytes. A `FuncData`
is required for all host functions in a store, including those inserted
from a `Linker` into a store used during linking. This means that
instantiation ends up creating a nontrivial number of these types and
pushing them into the store. Looking at some profiles there were some
surprisingly expensive movements of `FuncData` from the stack to a
vector for moves-by-value generated by Rust. Shrinking this type enables
more efficient code to be generated and additionally means less storage
is needed in a store's function array.

For instantiating the spidermonkey and rustpython modules this improves
instantiation by 10% since they each import a fair number of host
functions and the speedup here is relative to the number of items
imported.

* Use `ptr::copy_nonoverlapping` during initialization

Prevoiusly `ptr::copy` was used for copying imports into place which
translates to `memmove`, but `ptr::copy_nonoverlapping` can be used here
since it's statically known these areas don't overlap. While this
doesn't end up having a performance difference it's something I kept
noticing while looking at the disassembly of `initialize_vmcontext` so I
figured I'd go ahead and implement.

* Indirect shared signature ids in the VMContext

This commit is a small improvement for the instantiation time of modules
by avoiding copying a list of `VMSharedSignatureIndex` entries into each
`VMContext`, instead building one inside of a module and sharing that
amongst all instances. This involves less lookups at instantiation time
and less movement of data during instantiation. The downside is that
type-checks on `call_indirect` now involve an additionally load, but I'm
assuming that these are somewhat pessimized enough as-is that the
runtime impact won't be much there.

For instantiation performance this is a 5-10% win with
rustpyhon/spidermonky instantiation. This should also reduce the size of
each `VMContext` for an instantiation since signatures are no longer
stored inline but shared amongst all instances with one module.

Note that one subtle change here is that the array of
`VMSharedSignatureIndex` was previously indexed by `TypeIndex`, and now
it's indexed by `SignaturedIndex` which is a deduplicated form of
`TypeIndex`. This is done because we already had a list of those lying
around in `Module`, so it was easier to reuse that than to build a
separate array and store it somewhere.

* Reserve space in `Store<T>` with `InstancePre`

This commit updates the instantiation process to reserve space in a
`Store<T>` for the functions that an `InstancePre<T>`, as part of
instantiation, will insert into it. Using an `InstancePre<T>` to
instantiate allows pre-computing the number of host functions that will
be inserted into a store, and by pre-reserving space we can avoid costly
reallocations during instantiation by ensuring the function vector has
enough space to fit everything during the instantiation process.

Overall this makes instantiation of rustpython/spidermonkey about 8%
faster locally.

* Fix tests

* Use checked arithmetic

crates/cranelift/src/func_environ.rs

@@ -1556,13 +1556,26 @@ impl<'module_environment> cranelift_wasm::FuncEnvironment for FuncEnvironment<'m
                 let sig_id_type = Type::int(u16::from(sig_id_size) * 8).unwrap();
                 let vmctx = self.vmctx(builder.func);
                 let base = builder.ins().global_value(pointer_type, vmctx);
-                let offset =
-                    i32::try_from(self.offsets.vmctx_vmshared_signature_id(ty_index)).unwrap();
 
-                // Load the caller ID.
+                // Load the caller ID. This requires loading the
+                // `*mut VMCallerCheckedAnyfunc` base pointer from `VMContext`
+                // and then loading, based on `SignatureIndex`, the
+                // corresponding entry.
                 let mut mem_flags = ir::MemFlags::trusted();
                 mem_flags.set_readonly();
-                let caller_sig_id = builder.ins().load(sig_id_type, mem_flags, base, offset);
+                let signatures = builder.ins().load(
+                    pointer_type,
+                    mem_flags,
+                    base,
+                    i32::try_from(self.offsets.vmctx_signature_ids_array()).unwrap(),
+                );
+                let sig_index = self.module.types[ty_index].unwrap_function();
+                let offset =
+                    i32::try_from(sig_index.as_u32().checked_mul(sig_id_type.bytes()).unwrap())
+                        .unwrap();
+                let caller_sig_id = builder
+                    .ins()
+                    .load(sig_id_type, mem_flags, signatures, offset);
 
                 // Load the callee ID.
                 let mem_flags = ir::MemFlags::trusted();