Read/Write mutable reference in serialize and deserialize_reader is unnecessary

[billythedummy]

Currently, the required method for BorshDeserialize has the following signature:

fn deserialize_reader<R: Read>(reader: &mut R) -> Result<Self>;

However, it can be changed to just:

fn deserialize_reader<R: Read>(reader: R) -> Result<Self>;

because Read has a blanket impl for mut references: https://docs.rs/borsh/latest/borsh/io/trait.Read.html#impl-Read-for-%26mut+R

Similarly, BorshSerialize can be changed from

fn serialize<W: Write>(&self, writer: &mut W) -> Result<()>;

to

fn serialize<W: Write>(&self, writer: W) -> Result<()>;

because Write has a blanket impl for mut references: https://docs.rs/borsh/latest/borsh/io/trait.Write.html#impl-Write-for-%26mut+W

This will be a breaking change

[billythedummy]

wrong repo, sorry. Moved to: near/borsh-rs#270