Add From and TryFrom implementations for Data

[Abestanis]

Sometimes a multi byte value is stored over a span of registers, e.g a 64bit value could be stored over 4 consecutive registers. When sending a request to write to such a register, one has to use WriteMultipleRegisters(Address, Data<'r>), which requires a data.

If the rust code is using the value as a 64 bit integer, it is quite cumbersome to convert it to &[u16] that is required by Data::from_words, the only way to construct a Data.

This PR proposes some TryFrom and From implementations to help with these situations. You can now write

Data::from(&0x12345678_u64.to_be_bytes())

Since From can only be implemented for even number of bytes, a helper macro is used to declare implementations up to [u8; 32] (similar how Default is implemented for arrays in the standard library).

One question: Why is constructing a zero length Data forbidden? I don't have a usecase, but should it be allowed?

[flosse]

I think that such auxiliary functions have nothing directly to do with Modbus and should rather be kept separate.

[Abestanis]

I think that such auxiliary functions have nothing directly to do with Modbus and should rather be kept separate.

What do you mean by that? modbus-core enforces the use of Data in WriteMultipleRegisters(Address, Data<'r>), and the data member of Data is crate private. There is no way of adding this functionality from outside of the crate, except for jumping trough a bunch of hoops and u16 conversions with Data::from_words.

[flosse]

Sorry, that was my bad, I had forgotten that the fields of Data are not public.
So yes, your suggestion could make sense 🤔
Give me another chance to think about it before I merge this PR.

[flosse]

Okay, so I don't have a better suggestion at the moment, and I don't see any other difficulties for now.