Produce type directly from std::io::Read

[rbran]

First of all, thanks for the crate, it's amazing.

While using it, I found that i keep writing something like this:

let mut value = [0u8; 4];
reader.read_exact(&mut value)?;
let (_, value) = DekuValue::from_bytes((&value, 0))?;

And I wish that I could do something like this instead:

let (_, value) = DekuValue::from_reader(reader)?;

Proposal: https://gist.github.com/rbran/e7814fd6694ee37d54243236b77ff3cf

[wcampbell0x2a]

There is an issue to add this, just haven't implemented it: #105