Add `dbg` directive

[jam1garner]

Purely out of curiosity of how an implementation would feel, I implemented a dbg directive.

To explain, here's the documentation for it:

---

The dbg directive prints out information to stderr to help with quick-and-dirty debugging.

The information printed out:

• Source filename
• Line number in source file
• Offset in reader (in hex)
• Contents of parsed value (using hex format when available)

#[derive(BinRead)]
struct DbgExample {
    first_value: u32,
    #[br(dbg)]
    second_value: u32,
}

Which will output something like...

[src/main.rs:5 | offset 0x4] second_value = 0xFFFE76

---

This is not necessarily something we need to add, just putting this here to (a) collect thoughts and (b) have an implementation if ever desired.

[csnover]

Just had to figure out a bug and dumping thoughts here about things a dbg implementation probably should have beyond what is in this PR:

• Show the position of the reader both before and after the read (if there is an IO error indicate that in place of the end-read position). This would be implicit in the start position of the next field, except that they can be padded/aligned and also the last field doesn’t have a next field
• Show padding/alignment operations when they occur
• Show errors when they occur
• Show any applied directive values, e.g. if someone has if(foo) show the value of foo, if someone has pad_size_to(foo) show the foo and what actually happened
• Recursion through inner types?

[csnover]

I do believe this was superseded by https://github.com/jam1garner/binrw/pull/162.