[release] v1.0.1

After the merge into OpenGothic a couple other issues were uncovered. This patch addresses these issues and contain a set of compatibility improvements and some performance upgrades. These are the changes made since v1.0.0:

Bugfixes

• [d9357f6] Previously, only the non-const version of script::find_symbol_by_name was case-insensitive. Now
  the const version is too.
• [7d87485] Non-free waypoints are now correctly included in the way_net::waypoint lookup
• [c8c2294] Return null-instances from the default VM external set by vm::register_default_external if needed
• [ce320ad] If a default external is set the stack guard in the opcode::be branch of vm::exec is now inhibited
  preventing stack corruption
• [272c2a3] The stack guard for the opcode::bl instruction in vm::exec used sym as uninitialized which caused VM
  crashes if any script function was overridden
• [612b078] Fix incorrect usage of std::isspace in multiple locations which could cause crashes using compilers which
  treat char as signed

Performance

• [88c43eb] Improve archive_reader::read_object_begin performance by up to 30% by using sscanf instead
  of std::stringstream internally

Misc

• [52e3136, 21d59b8, 6bc9967] Allow using Git submodules in addition to CMake's FetchContent for resolving
  dependencies
• [ccf8ce6] Updated old SHA1 hash of the lexy dependency (thanks, @thokkat!)