fix(tomllib): 单引号处误用双引号字符串，改为字符字面量

char 形参与 char 比较位置写成「双引号包单引号」（实为 i8* 字符串）：形参处报 CastError（implicit pointer-to-integer），比较处被 C 哨兵规则静默 ptrtoint 成恒假（TOML 单引号字面量字符串根本解析不出来）。7 处统一改成字符字面量，与 lexer.vp 同款写法一致。

tomllib/__parser.vp

@@ -224,10 +224,10 @@ def _parse_basic_string(s: TomlState*) -> str:
 
 # Literal string '...': taken verbatim, no escapes.
 def _parse_literal_string(s: TomlState*) -> str:
-    if _peek(s) != "'": return None
+    if _peek(s) != '\'': return None
     _advance(s)
     start: sizet = s.pos
-    while s.pos < s.len and s.src[s.pos] != "'":
+    while s.pos < s.len and s.src[s.pos] != '\'':
         if _is_newline(s.src[s.pos]): return None
         s.pos += 1
     if s.pos >= s.len: return None
@@ -276,7 +276,7 @@ def _parse_multiline_literal(s: TomlState*) -> str:
     if _peek(s) == '\n': s.pos += 1
     start: sizet = s.pos
     while s.pos < s.len:
-        if s.src[s.pos] == "'" and s.pos + 2 < s.len and s.src[s.pos + 1] == "'" and s.src[s.pos + 2] == "'":
+        if s.src[s.pos] == '\'' and s.pos + 2 < s.len and s.src[s.pos + 1] == '\'' and s.src[s.pos + 2] == '\'':
             n: sizet = s.pos - start
             s.pos += 3
             return _copy_span(s.pool, s.src, start, n)
@@ -440,9 +440,9 @@ def _parse_value(s: TomlState*) -> JsonValue*:
         else: sv = _parse_basic_string(s)
         if sv == None: return None
         return string_val(s.pool, sv)
-    if ch == "'":
+    if ch == '\'':
         sv2: str = None
-        if _is_multiline_open(s, "'") != 0: sv2 = _parse_multiline_literal(s)
+        if _is_multiline_open(s, '\'') != 0: sv2 = _parse_multiline_literal(s)
         else: sv2 = _parse_literal_string(s)
         if sv2 == None: return None
         return string_val(s.pool, sv2)
@@ -468,8 +468,8 @@ def _parse_key_segment(s: TomlState*) -> str:
     if ch == '"':
         if _is_multiline_open(s, '"') != 0: return None
         return _parse_basic_string(s)
-    if ch == "'":
-        if _is_multiline_open(s, "'") != 0: return None
+    if ch == '\'':
+        if _is_multiline_open(s, '\'') != 0: return None
         return _parse_literal_string(s)
     if not _is_bare_key_char(ch): return None
     start: sizet = s.pos