fix(qr): Crystal use C FFI instead of qr-code shard

Crystal shards pre-cached during build aren't available for
single-file execution. Use @[Link("qrencode")] FFI instead.

test/qr.cr

@@ -1,5 +1,19 @@
-require "qr-code"
+@[Link("qrencode")]
+lib LibQRencode
+  struct QRcode
+    version : LibC::Int
+    width : LibC::Int
+    data : UInt8*
+  end
 
-qr = QRCode.new("unsandbox-qr-ok")
-rows = qr.modules.size
-puts "QR:unsandbox-qr-ok:ROWS:#{rows}"
+  fun QRcode_encodeString(s : LibC::Char*, version : LibC::Int, level : LibC::Int, hint : LibC::Int, casesensitive : LibC::Int) : QRcode*
+  fun QRcode_free(qr : QRcode*)
+end
+
+qr = LibQRencode.QRcode_encodeString("unsandbox-qr-ok", 0, 1, 2, 1)
+if qr.null?
+  puts "QR encode failed"
+  exit 1
+end
+puts "QR:unsandbox-qr-ok:ROWS:#{qr.value.width}"
+LibQRencode.QRcode_free(qr)