Enum: use signed instead of unsigned int for enum representation

This hits upon a larger issue. It doesn't seem fully clear what type a
compiler will use to represent an enum, but signed integer seems to be a
good enough solution as enum-constants (not the enum itself) are
defined as int signed. See following patch note

https://gcc.gnu.org/legacy-ml/gcc-patches/2000-07/msg00993.html

Signed-off-by: Sophie Tyalie <dev@flowerpot.me>

Author: tyalie

cstruct/base.py

@@ -94,10 +94,10 @@
 }
 
 ENUM_SIZE_TO_C_TYPE: Dict[int, str] = {
-    1: 'uint8',
-    2: 'uint16',
-    4: 'uint32',
-    8: 'uint64'
+    1: 'int8',
+    2: 'int16',
+    4: 'int32',
+    8: 'int64'
 }
 
 CHAR_ZERO = bytes('\0', 'ascii')