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Fixes, tests, and enhancements for grep (part 1)

See merge request !53

This doesn't do a lot for us here, but there are a lot more options
in even the most minimal of major grep implementations, so it makes
sense to switch over now before making things more complicated.

One thing we do get from this is combined short-form options (yay!).

This encompasses a few things:
1. Return 0 if there were matches and no errors (includes --help).
2. Return 1 if there were no matches (useful in scripts).
3. Return >1 if there were any errors, but wait to do so until all files
have been processed (useful when globbing over a busy directory, for
example).

Support for math functions

See merge request !52

Cleanup and modernization

See merge request !51

This does not call the closure on the success case.

Also ignore a couple clippy warnings.

Clippy suggested fixes, including number constant formatting.
Print out error on uptime parse problem.
Line up columns for right-side items, reducing repeated string
constants.
Use print!() and println!().

Replace instances of stdout.write(...).unwrap() and similar with
println!().  Ditto for stderr and eprintln!().

Update for current Rust.
Simplify logic, fix iteration over `String`.

Warnings and cleanup - 2nd try

See merge request !49