Support serialization with bincode v2

[vbkaisetsu]

This branch uses bincode for serialization and deserialization features of Model and Predictor.

The deserialization feature of the Predictor is unsafe and users should not load serialized data provided by thirdparty distributors, so the predict command does not support loading serialized Predictors.

I will add an example usage in another branch.

[kampersanda]

Thank you for your update.

Before carefully reviewing the code, I remember that bincode was removed once in #18 (of internal repo) because of the write/read speed.

With this modification, are there any major speed issues?

[vbkaisetsu]

Previously, I tried bincode v1.
bincode v2 is a little bit slower than our implementation, but the model size can be reduced.

w/o tags

bincode:            1,956,943 bytes
our implementation: 1,993,550 bytes

w/ tags

bincode:            2,834,891 bytes
our implementation: 3,014,008 bytes

[vbkaisetsu]

Since it is not very slow, I want to employ bincode v2 for ease of maintenance.