encoding: implement hex encoding (base16)

Implement hex encoding and decoding (base16) according to RFC 4648.
Add unit tests.

Link: https://www.rfc-editor.org/rfc/rfc4648

test/unit/stdlib/encoding/hex.c3

@@ -0,0 +1,45 @@
+module encoding::hex @test;
+import std::encoding::hex;
+
+struct TestCase
+{
+    char[] dec;
+    char[] enc;
+}
+
+TestCase[] tests = {
+	{"", ""},
+	{{'g'}, "67"},
+	{{0,1,2,3,4,5,6,7}, "0001020304050607"},
+	{{8,9,10,11,12,13,14,15}, "08090a0b0c0d0e0f"},
+	{{0xf0, 0xf1, 0xf2, 0xf3}, "f0f1f2f3"},
+	{{0xe3, 0xa1}, "e3a1"},
+	{{0xe3, 0xa1}, "E3A1"},
+};
+
+fn void! encode() {
+	usz n;
+	char[64] buf;
+	foreach (t : tests) {
+		n = hex::encode_bytes(t.dec, buf[..])!;
+		String want = ((String)t.enc).temp_ascii_to_lower();
+		assert(want == buf[:n], "encode failed: got: %s, want: %s", buf[:n], want);
+		@pool()
+		{
+			assert(want == hex::encode_temp(t.dec)!);
+		};
+	}
+}
+
+fn void! decode() {
+	usz n;
+	char[64] buf;
+	foreach (t : tests) {
+		n = hex::decode_bytes(t.enc, buf[..])!;
+		assert(t.dec == buf[:n], "decode failed: got: %s, want: %s", buf[:n], t.dec);
+		@pool()
+		{
+			assert(t.dec == hex::decode_temp(t.enc)!);
+		};
+	}
+}