Refactor runtime Table to support static storage.

This commit refactors `Table` in the runtime such that it can be created from a
pointer to existing table data.

The current `Vec` backing of the `Table` is considered to be "dynamic" storage.

This will be used for the upcoming pooling allocator where table memory is
managed externally to the instance.

The `table.copy` implementation was improved to use slice primitives for doing
the copying.

Fixes #983.

crates/runtime/src/instance/allocator.rs

@@ -290,7 +290,7 @@ impl OnDemandInstanceAllocator {
         let mut tables: PrimaryMap<DefinedTableIndex, _> =
             PrimaryMap::with_capacity(module.table_plans.len() - num_imports);
         for table in &module.table_plans.values().as_slice()[num_imports..] {
-            tables.push(Table::new(table));
+            tables.push(Table::new_dynamic(table));
         }
         tables.into_boxed_slice()
     }