tensor: support string encode/decode to a tensor about tensorflow-nodejs

[yorkie]

Interesting working with TF_StringEncode and TF_StringDecode function, also find the calls to TF_StringEncodedSize is 2 for normal usage. Anyway, it works by this commit 74e2426.

[yorkie]

Oh 74e2426 just are the partial of this issue, reopening this.

The full design of this string tensor is: a string tensor is an array of strings, and every elements are encoded with the function TF_StringEncode, and the whole buffer structure are: elements offsets + elements buffer.

For example, if we want to encode a string: [ "foobar" ], it's:

offsets[1] + [ encode("foobar") ]

and [ "foobar", "beep" ]:

offsets[1, x] + [ encode("foobar"), encode("beep") ]

In offsets part, it's an array, too. every element is an uint64 to represent the offset of its elements in the whole buffer. That means the offsets length is the elements number.