Optimize table.init instruction and instantiation (#2847)

* Optimize `table.init` instruction and instantiation

This commit optimizes table initialization as part of instance
instantiation and also applies the same optimization to the `table.init`
instruction. One part of this commit is to remove some preexisting
duplication between instance instantiation and the `table.init`
instruction itself, after this the actual implementation of `table.init`
is optimized to effectively have fewer bounds checks in fewer places and
have a much tighter loop for instantiation.

A big fallout from this change is that memory/table initializer offsets
are now stored as `u32` instead of `usize` to remove a few casts in a
few places. This ended up requiring moving some overflow checks that
happened in parsing to later in code itself because otherwise the wrong
spec test errors are emitted during testing. I've tried to trace where
these can possibly overflow but I think that I managed to get
everything.

In a local synthetic test where an empty module with a single 80,000
element initializer this improves total instantiation time by 4x (562us
=> 141us)

* Review comments

crates/runtime/src/table.rs

@@ -7,7 +7,7 @@ use crate::{ResourceLimiter, Trap, VMExternRef};
 use anyhow::{bail, Result};
 use std::cell::{Cell, RefCell};
 use std::cmp::min;
-use std::convert::TryInto;
+use std::convert::{TryFrom, TryInto};
 use std::ops::Range;
 use std::ptr;
 use std::rc::Rc;
@@ -212,6 +212,32 @@ impl Table {
         }
     }
 
+    /// Fill `table[dst..]` with values from `items`
+    ///
+    /// Returns a trap error on out-of-bounds accesses.
+    pub fn init_funcs(
+        &self,
+        dst: u32,
+        items: impl ExactSizeIterator<Item = *mut VMCallerCheckedAnyfunc>,
+    ) -> Result<(), Trap> {
+        assert!(self.element_type() == TableElementType::Func);
+
+        self.with_elements_mut(|elements| {
+            let elements = match elements
+                .get_mut(usize::try_from(dst).unwrap()..)
+                .and_then(|s| s.get_mut(..items.len()))
+            {
+                Some(elements) => elements,
+                None => return Err(Trap::wasm(ir::TrapCode::TableOutOfBounds)),
+            };
+
+            for (item, slot) in items.zip(elements) {
+                *slot = item as *mut u8;
+            }
+            Ok(())
+        })
+    }
+
     /// Fill `table[dst..dst + len]` with `val`.
     ///
     /// Returns a trap error on out-of-bounds accesses.