Make Clippy happier, minor cleanups

[nyurik]

I ran cargo clippy and addressed some issues it raised, while leaving other issues for another time. Hopefully this will make this code a bit easier for newcomers to read.

Biggest changes:

• foo.len() > 0 ➡ !foo.is_empty()
• return x; ➡ x
• { foo: foo } ➡ { foo }
• adding a few Default trait implementations
• some match statement cleanup

Also, fixed two ffi::uInt::max_value() ➡ ffi::uInt::MAX due to deprecation.

Biggest TODOs (please give feedback if they should be fixed too):

• Clippy has complained about many foo >> 0 -- which is a noop that might confuse (?)
• some other corner cases that I don't know enough about

[nyurik]

@alexcrichton thx for merging! Do you have any thoughts about the TODO in the comment?

[alexcrichton]

Sorry no, I can perhaps offer my thoughts on a case-by-case basis but I don't run clippy myself.