feat(lexer, preprocessor): replace cstring conversion with copy and refactor macro expansion

- Replace `scc_cstring_from_cstr(scc_cstring_as_cstr(...))` with `scc_cstring_copy()` in lexer to fix memory leaks
- Extract macro expansion logic into separate `expand_macro.c` file
- Remove `expand_stack` parameter from `scc_pp_expand_macro()` function
- Add new parsing functions for macro replacement lists and arguments
- Add string utility functions for whitespace trimming and string joining
- Update memory stream documentation for clarity

libs/lexer/src/lexer.c

@@ -79,7 +79,7 @@ void scc_lexer_init(scc_lexer_t *lexer, scc_probe_stream_t *stream) {
     lexer->stream = stream;
     lexer->pos = scc_pos_init();
     // FIXME
-    lexer->pos.name = scc_cstring_from_cstr(scc_cstring_as_cstr(&stream->name));
+    lexer->pos.name = scc_cstring_copy(&stream->name);
 }
 
 #define set_err_token(token) ((token)->type = SCC_TOK_UNKNOWN)
@@ -135,7 +135,7 @@ static void parse_line(scc_lexer_t *lexer, scc_lexer_tok_t *token) {
     scc_lex_parse_skip_line(lexer->stream, &lexer->pos);
     token->loc.line = n;
     // FIXME memory leak
-    token->loc.name = scc_cstring_from_cstr(scc_cstring_as_cstr(&str));
+    token->loc.name = scc_cstring_copy(&str);
     scc_cstring_free(&str);
     return;
 SKIP_LINE: