py: Free non-interned strings in the parser when not needed.

mp_parse_node_free now frees the memory associated with non-interned
strings.  And the parser calls mp_parse_node_free when discarding a
non-used node (such as a doc string).

Also, the compiler now frees the parse tree explicitly just before it
exits (as opposed to relying on the caller to do this).

Addresses issue #708 as best we can.

bare-arm/main.c

@@ -35,7 +35,6 @@ void do_str(const char *src) {
     qstr source_name = mp_lexer_source_name(lex);
     mp_lexer_free(lex);
     mp_obj_t module_fun = mp_compile(pn, source_name, MP_EMIT_OPT_NONE, true);
-    mp_parse_node_free(pn);
 
     if (module_fun == mp_const_none) {
         // compile error

py/builtinevex.c

@@ -63,7 +63,6 @@ STATIC mp_obj_t parse_compile_execute(mp_obj_t o_in, mp_parse_input_kind_t parse
 
     // compile the string
     mp_obj_t module_fun = mp_compile(pn, source_name, MP_EMIT_OPT_NONE, false);
-    mp_parse_node_free(pn);
 
     if (module_fun == mp_const_none) {
         // TODO handle compile error correctly

py/builtinimport.c

@@ -141,7 +141,6 @@ STATIC void do_load(mp_obj_t module_obj, vstr_t *file) {
 
     // compile the imported script
     mp_obj_t module_fun = mp_compile(pn, source_name, MP_EMIT_OPT_NONE, false);
-    mp_parse_node_free(pn);
 
     if (module_fun == mp_const_none) {
         // TODO handle compile error correctly

py/compile.c

@@ -3710,6 +3710,9 @@ mp_obj_t mp_compile(mp_parse_node_t pn, qstr source_file, uint emit_opt, bool is
 #endif
 #endif // !MICROPY_EMIT_CPYTHON
 
+    // free the parse tree
+    mp_parse_node_free(pn);
+
     // free the scopes
     mp_raw_code_t *outer_raw_code = module_scope->raw_code;
     for (scope_t *s = module_scope; s;) {

py/compile.h

@@ -33,4 +33,5 @@ enum {
     MP_EMIT_OPT_ASM_THUMB,
 };
 
+// the compiler will free the parse tree (pn) before it returns
 mp_obj_t mp_compile(mp_parse_node_t pn, qstr source_file, uint emit_opt, bool is_repl);

py/parse.c

@@ -179,6 +179,7 @@ void mp_parse_node_free(mp_parse_node_t pn) {
         mp_uint_t n = MP_PARSE_NODE_STRUCT_NUM_NODES(pns);
         mp_uint_t rule_id = MP_PARSE_NODE_STRUCT_KIND(pns);
         if (rule_id == RULE_string) {
+            m_del(char, (char*)pns->nodes[0], (mp_uint_t)pns->nodes[1]);
             return;
         }
         bool adjust = ADD_BLANK_NODE(rule_id);
@@ -562,8 +563,8 @@ mp_parse_node_t mp_parse(mp_lexer_t *lex, mp_parse_input_kind_t input_kind, mp_p
                 if (input_kind != MP_PARSE_SINGLE_INPUT && rule->rule_id == RULE_expr_stmt && peek_result(&parser, 0) == MP_PARSE_NODE_NULL) {
                     mp_parse_node_t p = peek_result(&parser, 1);
                     if ((MP_PARSE_NODE_IS_LEAF(p) && !MP_PARSE_NODE_IS_ID(p)) || MP_PARSE_NODE_IS_STRUCT_KIND(p, RULE_string)) {
-                        pop_result(&parser);
-                        pop_result(&parser);
+                        pop_result(&parser); // MP_PARSE_NODE_NULL
+                        mp_parse_node_free(pop_result(&parser)); // RULE_string
                         push_result_rule(&parser, rule_src_line, rules[RULE_pass_stmt], 0);
                         break;
                     }

qemu-arm/main.c

@@ -35,7 +35,6 @@ void do_str(const char *src) {
     qstr source_name = mp_lexer_source_name(lex);
     mp_lexer_free(lex);
     mp_obj_t module_fun = mp_compile(pn, source_name, MP_EMIT_OPT_NONE, true);
-    mp_parse_node_free(pn);
 
     if (module_fun == mp_const_none) {
         // compile error

qemu-arm/test_main.c

@@ -38,7 +38,6 @@ inline void do_str(const char *src) {
     qstr source_name = mp_lexer_source_name(lex);
     mp_lexer_free(lex);
     mp_obj_t module_fun = mp_compile(pn, source_name, MP_EMIT_OPT_NONE, true);
-    mp_parse_node_free(pn);
 
     if (module_fun == mp_const_none) {
         tt_abort_msg("Computer error");

stmhal/pyexec.c

@@ -71,7 +71,6 @@ bool parse_compile_execute(mp_lexer_t *lex, mp_parse_input_kind_t input_kind, bo
     mp_lexer_free(lex);
 
     mp_obj_t module_fun = mp_compile(pn, source_name, MP_EMIT_OPT_NONE, is_repl);
-    mp_parse_node_free(pn);
 
     if (module_fun == mp_const_none) {
         return false;