Add serialization and deserialization functions

[vbkaisetsu]

This branch adds the following serialization and deserialization functions for convenience.

• serialize()
• serialize_to_vec()
• deserialize_unchecked()
• deserialize_from_slice_unchecked()

Deserialization functions do not validate the given automaton, so these are defined as unsafe functions.
It means that the deserialization should only be used when the source of the data is completely trusted.

serialize() and deserialize_unchecked() use traits under std::io, so these functions are unavailable in the no_std environment. For bare-metal programming, they must use serialize_to_vec() and deserialize_from_slice_unchecked() instead.

[kampersanda]

Thank you for the implementation!

I also think that employing unsafe declaration is a reasonable approach to implement deserialization.

One concern is that Daachorse, or its data structure, is expected to be updated, resulting in some difficulty to manage versions of models.
So, it would be better to add validation of the model version.
That is, I suggest that the serialization functions embed the current package version into the head of a file, and the deserialization functions validate the version.
The package version can be known through env!("CARGO_PKG_VERSION").
https://doc.rust-lang.org/cargo/reference/environment-variables.html

What do you think? @vbkaisetsu

[vbkaisetsu]

unsafe functions should take a correct value, so I think any validation including version checking is unnecessary.

The intended use case is as follows:

• Implementing pattern match on small devices that do not have a power to build automata.
• Transporting automata on a single program.
• Building automata using build.rs

This feature should not be used for distributing pre-built automata because users should not trust them.

[vbkaisetsu]

@kampersanda

[kampersanda]

I agree with you.