Pre-allocate vectors in SSZ decoding (sigp#3417)

## Issue Addressed

Fixes a potential regression in memory fragmentation identified by @paulhauner here: sigp#3371 (comment).

## Proposed Changes

Immediately allocate a vector with sufficient size to hold all decoded elements in SSZ decoding. The `size_hint` is derived from the range iterator here:

https://github.com/sigp/lighthouse/blob/2983235650811437b44199f9c94e517e948a1e9b/consensus/ssz/src/decode/impls.rs#L489

## Additional Info

I'd like to test this out on some infra for a substantial duration to see if it affects total fragmentation.

consensus/ssz/src/decode/try_from_iter.rs

@@ -29,11 +29,18 @@ pub trait TryFromIter<T>: Sized {
 impl<T> TryFromIter<T> for Vec<T> {
     type Error = Infallible;
 
-    fn try_from_iter<I>(iter: I) -> Result<Self, Self::Error>
+    fn try_from_iter<I>(values: I) -> Result<Self, Self::Error>
     where
         I: IntoIterator<Item = T>,
     {
-        Ok(Self::from_iter(iter))
+        // Pre-allocate the expected size of the Vec, which is parsed from the SSZ input bytes as
+        // `num_items`. This length has already been checked to be less than or equal to the type's
+        // maximum length in `decode_list_of_variable_length_items`.
+        let iter = values.into_iter();
+        let (_, opt_max_len) = iter.size_hint();
+        let mut vec = Vec::with_capacity(opt_max_len.unwrap_or(0));
+        vec.extend(iter);
+        Ok(vec)
     }
 }