Improve the readability of stack traces originating in the runtime's main function

[Duckwhale]

Goals:

• [x] Stack traces should indicate a human-readable entry point for main.c at the highest level of the stack

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Right now this is how they appear:

With tiny adjustments, the output can be turned into something like this:

The parentheses are consistent with what Lua (and LuaJIT) use when running code, but a bit more verbose to hopefully be less confusing. The other issues with readability of stack traces can be fixed by including debug symbols in the object files.

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While experimenting, I just swapped the initialization code with this:

#define S1(x) #x
#define S2(x) S1(x)
#define LOCATION __FILE__ ":" S2(__LINE__)
    const char* entry_point = "return require('init')(...)";

    if (luaL_loadbuffer(L, entry_point, strlen(entry_point), "=(Lua entry point, at " LOCATION ")")) {
        fprintf(stderr, "%s\n", lua_tostring(L, -1));
        vm_release(L);
        return -1;
    }

#undef LOCATION

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Split off from #4 because this is probably "good enough" for the time being and should be committed separately.