fix: handle small integer types in variadic args

In C's variadic functions, small integer types (char, short) are promoted to int
when passed as arguments. This change fixes the VaArgSafe trait implementation
issues by:

- Reading char/short values as i32/u32 first, then converting to i8/i16/u8/u16
- Fixing the %c format specifier to read u32 and convert to u8
- Maintaining C's integer promotion behavior while satisfying Rust's type safety

src/parser.rs

@@ -74,8 +74,8 @@ impl Length {
     unsafe fn parse_signed(self, args: &mut VaList) -> SignedInt {
         match self {
             Length::Int => SignedInt::Int(args.arg()),
-            Length::Char => SignedInt::Char(args.arg()),
-            Length::Short => SignedInt::Short(args.arg()),
+            Length::Char => SignedInt::Char(args.arg::<i32>() as i8),
+            Length::Short => SignedInt::Short(args.arg::<i32>() as i16),
             Length::Long => SignedInt::Long(args.arg()),
             Length::LongLong => SignedInt::LongLong(args.arg()),
             // for some reason, these exist as different options, yet produce the same output
@@ -85,8 +85,8 @@ impl Length {
     unsafe fn parse_unsigned(self, args: &mut VaList) -> UnsignedInt {
         match self {
             Length::Int => UnsignedInt::Int(args.arg()),
-            Length::Char => UnsignedInt::Char(args.arg()),
-            Length::Short => UnsignedInt::Short(args.arg()),
+            Length::Char => UnsignedInt::Char(args.arg::<u32>() as u8),
+            Length::Short => UnsignedInt::Short(args.arg::<u32>() as u16),
             Length::Long => UnsignedInt::Long(args.arg()),
             Length::LongLong => UnsignedInt::LongLong(args.arg()),
             // for some reason, these exist as different options, yet produce the same output
@@ -188,7 +188,7 @@ pub unsafe fn format(
                     format: DoubleFormat::Hex.set_upper(ch.is_ascii_uppercase()),
                 },
                 b's' => Specifier::String(CStr::from_ptr(args.arg())),
-                b'c' => Specifier::Char(args.arg()),
+                b'c' => Specifier::Char(args.arg::<u32>() as u8),
                 b'p' => Specifier::Pointer(args.arg()),
                 b'n' => Specifier::WriteBytesWritten(written, args.arg()),
                 _ => return -1,