Implement data.drop and memory.init and get the rest of the bulk memory spec tests passing (#1264)

* Enable the already-passing `bulk-memoryoperations/imports.wast` test

* Implement support for the `memory.init` instruction and passive data

This adds support for passive data segments and the `memory.init` instruction
from the bulk memory operations proposal. Passive data segments are stored on
the Wasm module and then `memory.init` instructions copy their contents into
memory.

* Implement the `data.drop` instruction

This allows wasm modules to deallocate passive data segments that it doesn't
need anymore. We keep track of which segments have not been dropped on an
`Instance` and when dropping them, remove the entry from the instance's hash
map. The module always needs all of the segments for new instantiations.

* Enable final bulk memory operations spec test

This requires special casing an expected error message for an `assert_trap`,
since the expected error message contains the index of an uninitialized table
element, but our trap implementation doesn't save that diagnostic information
and shepherd it out.

crates/runtime/src/vmcontext.rs

@@ -565,6 +565,10 @@ impl VMBuiltinFunctionsArray {
             wasmtime_memory_fill as usize;
         ptrs[BuiltinFunctionIndex::get_imported_memory_fill_index().index() as usize] =
             wasmtime_imported_memory_fill as usize;
+        ptrs[BuiltinFunctionIndex::get_memory_init_index().index() as usize] =
+            wasmtime_memory_init as usize;
+        ptrs[BuiltinFunctionIndex::get_data_drop_index().index() as usize] =
+            wasmtime_data_drop as usize;
 
         debug_assert!(ptrs.iter().cloned().all(|p| p != 0));