One may encode a non-printable string under the MsgPack type MP_STR.
In fact, there's no way to encode binary data as MP_BIN from Lua so
users often do that, which breaks our gdb extension. Let's encode a
string in base64 if unicode() raises an error, like we do with MP_BIN.

While we are at it,
 - escape double quotes in printable strings;
 - fix MP_BIN encoding (b64encode takes and returns bytes).
 - don't use YAML !!binary prefix before base64 data - use bin: and str:
   instead.

Here's what it looks like:

```
(gdb) p *box_tuple_last
  <...>
  data = ["\"тест\\foo\"", str:/w==, bin:/w==]}
```

The tuple was created with the following command:

```
tarantool> msgpack = require('msgpack')
---
...

tarantool> box.tuple.new(
         >   msgpack.object_from_raw('\xae"тест\\foo"'), -- MP_STR unicode
         >   msgpack.object_from_raw('\xa1\xff'),        -- MP_STR binary
         >   msgpack.object_from_raw('\xc4\x01\xff'))    -- MP_BIN
---
- ['"тест\foo"', !!binary /w==, !!binary /w==]
...
```

Follow-up commit 08a171a4 ("gdb: support unicode in MP_STR type of
MsgPack").

NO_DOC=gdb extension
NO_TEST=gdb extension
NO_CHANGELOG=gdb extension