port rpcgen to mercury for hg_proc_cb_t function generation

Port sunrpc rpcgen to mercury to allow mercury users to generate
hg_proc_cb_t functions from .x files (as an additional alternative
to hand coding or using the boost preprocessor macros).  The
mercury version of rpcgen (hg_rpcgen) parses standard .x files
(though it ignores the 'program' section) and emits code that
uses the mercury proc API.

The port includes:
  - rpcgen itself, in util/rpc_gen (hg_info is moved to util/hg_info)
  - mercury proc versions of helper xdr functions like xdr_array()
    in src/proc_extra.  we update the legacy string functions in
    src/proc_extra to use hg_proc_string() for a backend.
  - a new unit test in Testing/unit/hg/test_hg_rpcgen.c

The unit test's input .x file (Testing/unit/hg/test_hg_rpcgen.x)
also documents the the .x file syntax.

Note that the hg_rpcgen reinitialize() code has been updated to
free() previously allocated parse tree memory rather than discard
(leak) it.  To do this, the char* strings in parse.h were changed
to tokens (since the token type/kind value can be used to determine
if the token string was allocated with malloc() or not).

Testing/unit/hg/CMakeLists.txt

@@ -27,16 +27,30 @@ endif()
 function(build_mercury_test test_name)
   if(${CMAKE_VERSION} VERSION_GREATER 3.12)
     add_executable(hg_test_${test_name} test_${test_name}.c)
-    target_link_libraries(hg_test_${test_name} mercury_unit)
+    target_link_libraries(hg_test_${test_name} mercury_unit ${ARGN})
   else()
     add_executable(hg_test_${test_name} test_${test_name}.c mercury_unit.c mercury_rpc_cb.c)
-    target_link_libraries(hg_test_${test_name} mercury_test_common)
+    target_link_libraries(hg_test_${test_name} mercury_test_common ${ARGN})
   endif()
   if(MERCURY_ENABLE_COVERAGE)
     set_coverage_flags(hg_test_${test_name})
   endif()
 endfunction()
 
+# add ${test_xname} lib target using hg_rpcgen on file ${test_xname}.x
+function(build_mercury_rpcgen_procs test_xname)
+  add_custom_command(
+    OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/${test_xname}.h
+           ${CMAKE_CURRENT_BINARY_DIR}/${test_xname}_proc.c
+    COMMAND ${CMAKE_BINARY_DIR}/bin/hg_rpcgen
+                  ${CMAKE_CURRENT_SOURCE_DIR}/${test_xname}.x
+    DEPENDS ${test_xname}.x)
+    add_library(${test_xname} ${test_xname}_proc.c)
+    target_link_libraries(${test_xname} mercury)
+    target_include_directories(${test_xname}
+      PUBLIC ${CMAKE_CURRENT_BINARY_DIR})
+endfunction()
+
 macro(add_mercury_test test_name comm protocol busy parallel self scalable
   ignore_server_err)
   # Set full test name
@@ -222,9 +236,13 @@ build_mercury_test(bulk)
 build_mercury_test(lookup)
 build_mercury_test(proc)
 
+build_mercury_rpcgen_procs(test_hg_rpcgen)
+build_mercury_test(hg_rpcgen test_hg_rpcgen)
+
 build_mercury_test(kill)
 
 add_mercury_test_standalone(proc)
+add_mercury_test_standalone(hg_rpcgen)
 
 add_mercury_test_comm_all(rpc)
 add_mercury_test_comm_all(bulk)

Testing/unit/hg/mercury_rpc_cb.c

@@ -188,13 +188,11 @@ HG_TEST_RPC_CB(hg_test_rpc_open, handle)
     hg_size_t payload_size = HG_Get_input_payload_size(handle);
 #ifdef HG_HAS_XDR
     size_t expected_string_payload_size =     /* note xdr rounding rules */
-        sizeof(uint64_t) +                    /* length */
-        RNDUP(strlen(HG_TEST_RPC_PATH) + 1) + /* string data, inc \0 at end */
-        RNDUP(sizeof(uint8_t)) +              /* is_const */
-        RNDUP(sizeof(uint8_t));               /* is_owned */
+        sizeof(uint32_t) +                    /* length */
+        RNDUP(strlen(HG_TEST_RPC_PATH));      /* string data (no \0) */
 #else
     size_t expected_string_payload_size =
-        strlen(HG_TEST_RPC_PATH) + sizeof(uint64_t) + 3;
+        sizeof(uint32_t) + strlen(HG_TEST_RPC_PATH);  /* len + data */
 #endif
 
     HG_TEST_CHECK_ERROR(