Account for char possibly being unsigned

This sounds rather insane to me but I've managed to show that `(char)
-1` is converted to 255 on some platforms. This was reproduced on
ppc64el via Qemu on OS X. A simple program that does `fprintf(stderr,
"%d\r\n", (char) -1);` prints 255 to the console. Rather than rely on
the signedness of a char I've just updated things to use an unsigned
char (which hopefully is never signed) and replaced -1 with 255 for the
sentinel value when converting hex values.

Thanks to Balint Reczey (@rbalint) for the report.

Fixes #74

c_src/utf8.c

@@ -3,15 +3,23 @@
 #include "jiffy.h"
 #include <stdio.h>
 
-static const char hexvals[256] = {
-    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-     0,  1,  2,  3,  4,  5,  6,  7,  8,  9, -1, -1, -1, -1, -1, -1,
-    -1, 10, 11, 12, 13, 14, 15, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-    -1, 10, 11, 12, 13, 14, 15, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1
+static const unsigned char hexvals[256] = {
+    255, 255, 255, 255, 255, 255, 255, 255,
+    255, 255, 255, 255, 255, 255, 255, 255,
+    255, 255, 255, 255, 255, 255, 255, 255,
+    255, 255, 255, 255, 255, 255, 255, 255,
+    255, 255, 255, 255, 255, 255, 255, 255,
+    255, 255, 255, 255, 255, 255, 255, 255,
+      0,   1,   2,   3,   4,   5,   6,   7,
+      8,   9, 255, 255, 255, 255, 255, 255,
+    255,  10,  11,  12,  13,  14,  15, 255,
+    255, 255, 255, 255, 255, 255, 255, 255,
+    255, 255, 255, 255, 255, 255, 255, 255,
+    255, 255, 255, 255, 255, 255, 255, 255,
+    255,  10,  11,  12,  13,  14,  15, 255,
+    255, 255, 255, 255, 255, 255, 255, 255,
+    255, 255, 255, 255, 255, 255, 255, 255,
+    255, 255, 255, 255, 255, 255, 255, 255
 };
 
 static const char hexdigits[16] = {
@@ -27,10 +35,10 @@ int_from_hex(const unsigned char* p)
     unsigned char* h = (unsigned char*) p;
     int ret;
 
-    if(hexvals[*(h+0)] < 0) return -1;
-    if(hexvals[*(h+1)] < 0) return -1;
-    if(hexvals[*(h+2)] < 0) return -1;
-    if(hexvals[*(h+3)] < 0) return -1;
+    if(hexvals[*(h+0)] == 255) return -1;
+    if(hexvals[*(h+1)] == 255) return -1;
+    if(hexvals[*(h+2)] == 255) return -1;
+    if(hexvals[*(h+3)] == 255) return -1;
 
     ret = (hexvals[*(h+0)] << 12)
         + (hexvals[*(h+1)] << 8)