xC-gen-proto: reduce enums to single bytes

That's already way more than we can possibly use.

xC-gen-proto.awk

@@ -15,7 +15,7 @@
 # Booleans are one byte each.
 # Strings must be valid UTF-8, use u8<> to lift that restriction.
 # String and array lengths are encoded as u32.
-# Enumeration values automatically start at 1, and are encoded as i32.
+# Enumeration values automatically start at 1, and are encoded as i8.
 # Any struct or union field may be a variable-length array.
 #
 # Message framing is done externally, but also happens to prefix u32 lengths.
@@ -189,6 +189,8 @@ function defenum(    name, ident, value, cg) {
 			value = readnumber()
 		if (!value)
 			fatal("enumeration values cannot be zero")
+		if (value < -128 || value > 127)
+			fatal("enumeration value out of range")
 		expect(accept(","))
 		append(EnumValues, name, SUBSEP ident)
 		if (EnumValues[name, ident]++)