libslic3r: compare preset json keys without the locale

Reading one preset file tries a dozen key names against every key in it with
boost::iequals, which compares through std::locale(). Constructing that takes a
lock the whole process shares in the MSVC runtime, so it is both the bulk of the
work in a preset file's parse and the reason two threads cannot do it at once.

Comparing the bytes instead takes loading 328 user presets from 129 to 105 ms
one at a time, and the same files spread across threads from 132 to 27 ms. The
first launch after an update, when the system profiles are parsed from JSON
rather than served from their cache, goes through the same function.

src/libslic3r/Config.cpp

@@ -849,6 +849,19 @@ ConfigSubstitutions ConfigBase::load_from_json(const std::string &file, ForwardC
     return std::move(substitutions_ctxt.substitutions);
 }
 
+// Case-insensitive compare of a JSON key against a fixed ASCII one, without
+// boost::iequals, whose std::locale() takes a lock the whole process shares in the
+// MSVC runtime.
+static bool ascii_iequals(const std::string &key, const char *literal)
+{
+    auto lower = [](char c) { return (c >= 'A' && c <= 'Z') ? char(c - 'A' + 'a') : c; };
+    size_t i = 0;
+    for (; i < key.size() && literal[i] != '\0'; ++ i)
+        if (lower(key[i]) != lower(literal[i]))
+            return false;
+    return i == key.size() && literal[i] == '\0';
+}
+
 int ConfigBase::load_from_json(const std::string &file, ConfigSubstitutionContext& substitution_context, bool load_inherits_to_config, std::map<std::string, std::string>& key_values, std::string& reason)
 {
     json j;
@@ -916,44 +929,44 @@ int ConfigBase::load_from_json(const std::string &file, ConfigSubstitutionContex
         }
         //parse the json elements
         for (auto it = j.begin(); it != j.end(); it++) {
-            if (boost::iequals(it.key(),BBL_JSON_KEY_VERSION)) {
+            if (ascii_iequals(it.key(), BBL_JSON_KEY_VERSION)) {
                 key_values.emplace(BBL_JSON_KEY_VERSION, it.value());
             }
-            else if (boost::iequals(it.key(), BBL_JSON_KEY_IS_CUSTOM)) {
+            else if (ascii_iequals(it.key(), BBL_JSON_KEY_IS_CUSTOM)) {
                 //skip it
             }
-            else if (boost::iequals(it.key(), BBL_JSON_KEY_NAME)) {
+            else if (ascii_iequals(it.key(), BBL_JSON_KEY_NAME)) {
                 key_values.emplace(BBL_JSON_KEY_NAME, it.value());
                 if (it.value() == "project_settings")
                     is_project_settings = true;
             }
-            else if (boost::iequals(it.key(), BBL_JSON_KEY_URL)) {
+            else if (ascii_iequals(it.key(), BBL_JSON_KEY_URL)) {
                 key_values.emplace(BBL_JSON_KEY_URL, it.value());
             }
-            else if (boost::iequals(it.key(), BBL_JSON_KEY_TYPE)) {
+            else if (ascii_iequals(it.key(), BBL_JSON_KEY_TYPE)) {
                 key_values.emplace(BBL_JSON_KEY_TYPE, it.value());
             }
-            else if (boost::iequals(it.key(), BBL_JSON_KEY_SETTING_ID)) {
+            else if (ascii_iequals(it.key(), BBL_JSON_KEY_SETTING_ID)) {
                 key_values.emplace(BBL_JSON_KEY_SETTING_ID, it.value());
             }
-            else if (boost::iequals(it.key(), BBL_JSON_KEY_FILAMENT_ID)) {
+            else if (ascii_iequals(it.key(), BBL_JSON_KEY_FILAMENT_ID)) {
                 key_values.emplace(BBL_JSON_KEY_FILAMENT_ID, it.value());
             }
-            else if (boost::iequals(it.key(), BBL_JSON_KEY_FROM)) {
+            else if (ascii_iequals(it.key(), BBL_JSON_KEY_FROM)) {
                 key_values.emplace(BBL_JSON_KEY_FROM, it.value());
             }
-            else if (boost::iequals(it.key(), BBL_JSON_KEY_DESCRIPTION)) {
+            else if (ascii_iequals(it.key(), BBL_JSON_KEY_DESCRIPTION)) {
                 key_values.emplace(BBL_JSON_KEY_DESCRIPTION, it.value());
             }
-            else if (boost::iequals(it.key(), BBL_JSON_KEY_INSTANTIATION)) {
+            else if (ascii_iequals(it.key(), BBL_JSON_KEY_INSTANTIATION)) {
                 key_values.emplace(BBL_JSON_KEY_INSTANTIATION, it.value());
             }
-            else if (!load_inherits_to_config && boost::iequals(it.key(), BBL_JSON_KEY_INHERITS)) {
+            else if (!load_inherits_to_config && ascii_iequals(it.key(), BBL_JSON_KEY_INHERITS)) {
                 key_values.emplace(BBL_JSON_KEY_INHERITS, it.value());
             }
-            else if (!load_inherits_to_config && boost::iequals(it.key(), BBL_JSON_KEY_INCLUDES)) {
+            else if (!load_inherits_to_config && ascii_iequals(it.key(), BBL_JSON_KEY_INCLUDES)) {
                 key_values.emplace(BBL_JSON_KEY_INCLUDES, it.value().dump());
-            } else if (boost::iequals(it.key(), ORCA_JSON_KEY_RENAMED_FROM)) {
+            } else if (ascii_iequals(it.key(), ORCA_JSON_KEY_RENAMED_FROM)) {
                 key_values.emplace(ORCA_JSON_KEY_RENAMED_FROM, it.value());
             } else {
                 t_config_option_key opt_key = it.key();