tuple: hide internal functions from box.tuple.*

box.tuple.bless, .encode, and .is are internal. Their
behaviour is not documented, and they may omit some checks for the
sake of speed, and can crash if used without thinking.

Nonetheless, despite they are not documented, curious users could
notice them in box.tuple.* output via autocompletion, for example.
And they could try to use them. This is not ok.

box.tuple.bless() being called by a user leads either to a crash,
or to a leak (because it is basically tuple reference counter
increment).

box.tuple.encode() is kind of a wrapper around msgpack, and users
should not touch it. It may change, may be removed. And is just
makes no sense except some rare cases in schema.lua.

bless() and encode() were used in schema.lua only, so the patch
simply moves them to box.internal.tuple.

box.tuple.is() is kept as is, because
- this is used in the tests a lot;
- it is totally safe;
- that function actually makes sense, and some users could have
  already started using it.

There is no a test, since nothing to test - bless() is not
available for users anymore (assuming no one is brave enough to
rely on box.internal).

Closes #4684