Make parsing more memory flexible.

The parser attempts to allocate two large (~512 byte) chunks up
front. If it couldn't, then it would error out. This change will
cause it to try allocating half the previous attempt until its down
to two copies. This is ok upfront because later code checks bounds
and tries to extend the allocation if needed.

Author: tannewt

py/parse.c

@@ -742,11 +742,30 @@ mp_parse_tree_t mp_parse(mp_lexer_t *lex, mp_parse_input_kind_t input_kind) {
 
     parser.rule_stack_alloc = MICROPY_ALLOC_PARSE_RULE_INIT;
     parser.rule_stack_top = 0;
-    parser.rule_stack = m_new(rule_stack_t, parser.rule_stack_alloc);
+    parser.rule_stack = NULL;
+    while (parser.rule_stack_alloc > 1) {
+        parser.rule_stack = m_new_maybe(rule_stack_t, parser.rule_stack_alloc);
+        if (parser.rule_stack != NULL) {
+            break;
+        } else {
+            parser.rule_stack_alloc /= 2;
+        }
+    }
 
     parser.result_stack_alloc = MICROPY_ALLOC_PARSE_RESULT_INIT;
     parser.result_stack_top = 0;
-    parser.result_stack = m_new(mp_parse_node_t, parser.result_stack_alloc);
+    parser.result_stack = NULL;
+    while (parser.result_stack_alloc > 1) {
+        parser.result_stack = m_new_maybe(mp_parse_node_t, parser.result_stack_alloc);
+        if (parser.result_stack != NULL) {
+            break;
+        } else {
+            parser.result_stack_alloc /= 2;
+        }
+    }
+    if (parser.rule_stack == NULL || parser.result_stack == NULL) {
+        mp_raise_msg(&mp_type_MemoryError, "Unable to init parser");
+    }
 
     parser.lexer = lex;