Support conditional fields and variable-length arrays in the Wireshark Lua backend

Add an Emboss-expression -> Lua translator so the dissector backend can
handle constructs it previously skipped:

* Conditional (`if`) fields are emitted as `if <cond> then ... end`.
* Variable-length arrays (`T[n]`, with `n` a sibling field) and
  dynamically-located fields are emitted with the length/offset
  expression translated to Lua.

Sibling field values referenced by a condition, array length, or offset are
captured into `local val_*` reads. Fields whose governing expression can't be
translated are still skipped with a comment, so the generator always emits
valid Lua. Constant-only structs produce byte-identical output to before.

Add testdata/wireshark_dynamic.emb (and its golden), expression/conditional/
array unit tests, and a TShark smoke test that loads a generated dissector
and checks both branches of a conditional, length-prefixed message (the test
skips itself when tshark is not installed).

Author: AaronWebster

compiler/back_end/lua/BUILD

@@ -66,6 +66,19 @@ py_test(
     ],
 )
 
+# End-to-end check that runs a generated dissector under a real TShark.  It
+# skips itself when `tshark` / `text2pcap` aren't on PATH (e.g. in CI), so it is
+# safe to include in the default test set.
+py_test(
+    name = "tshark_smoke_test",
+    srcs = ["tshark_smoke_test.py"],
+    deps = [
+        ":dissector_generator",
+        "//compiler/front_end:glue",
+        "//compiler/util:test_util",
+    ],
+)
+
 py_library(
     name = "one_golden_test_lib",
     srcs = ["one_golden_test.py"],
@@ -109,3 +122,9 @@ lua_golden_test(
     emb_file = "//testdata:wireshark.emb",
     golden_file = "//testdata/golden_lua:wireshark.emb.lua",
 )
+
+lua_golden_test(
+    name = "wireshark_dynamic_golden_test",
+    emb_file = "//testdata:wireshark_dynamic.emb",
+    golden_file = "//testdata/golden_lua:wireshark_dynamic.emb.lua",
+)