wasm: Add support for passive data and element segments (#1389)

This is part of the bulk memory and reference types proposals.

cranelift/wasmtests/passive-data.wat

@@ -0,0 +1,11 @@
+(module
+  (data $passive "this is a passive data segment")
+
+  (func (export "init") (param i32 i32 i32)
+    local.get 0 ;; dst
+    local.get 1 ;; src
+    local.get 2 ;; cnt
+    memory.init $passive)
+
+  (func (export "drop")
+    data.drop $passive))