Implement compact file stacks in object files

Gets rid of `open_memstream`, enabling Windows compatibility again
Also fixes #491 as a nice bonus!

include/asm/lexer.h

@@ -43,6 +43,9 @@ static inline void lexer_SetGfxDigits(char const *digits)
 	gfxDigits = digits;
 }
 
+/*
+ * `path` is referenced, but not held onto..!
+ */
 struct LexerState *lexer_OpenFile(char const *path);
 struct LexerState *lexer_OpenFileView(char *buf, size_t size, uint32_t lineNo);
 void lexer_RestartRept(uint32_t lineNo);