fix: multi-arg calls, uninitialized symbols, param limit; add banner img; update docs

- Fix segfault from uninitialized Symbol structs in scope/hir_lower/sema
- Fix null ReadFile crash in Cli_MergeFileInto (missing stdlib files)
- Extend function params from 6 to 9 (bux_str_format needs 9 args)
- Add ExprList/HirArgList linked lists for >2 call arguments
- Add banner image to README
- Update docs: C backend is now primary (not QBE)

src_bux/lexer.bux

@@ -711,7 +711,7 @@ func Lexer_GetToken(lex: *Lexer, index: int) -> LexToken {
     if index >= 0 && index < lex.tokenCount {
         return lex.tokens[index];
     }
-    var empty: LexToken;
+    var empty: LexToken = LexToken { kind: tkEndOfFile, text: "", line: 0, column: 0 };
     return empty;
 }