feat(decode_string): decode binary string with NULs to Blob

Strings that previously decoded into a msgpack special for representing
BINs with NULs now convert to Blobs. It shouldn't be possible to decode
into this special anymore after this change?

Notably, Lua strings with NULs now convert to Blobs when passed to VimL.

test/functional/eval/msgpack_functions_spec.lua

@@ -364,8 +364,7 @@ describe('msgpack*() functions', function()
     command('let dumped = ["\\xC4\\x01\\n"]')
     command('let parsed = msgpackparse(dumped)')
     command('let dumped2 = msgpackdump(parsed)')
-    eq({{_TYPE={}, _VAL={'\n'}}}, eval('parsed'))
-    eq(1, eval('parsed[0]._TYPE is v:msgpack_types.binary'))
+    eq({'\000'}, eval('parsed'))
     eq(1, eval('dumped ==# dumped2'))
   end)