Refactor main.rs and fix clippy lints

Status: WIP

Completed:
* Fixed flagged clippy lints
* Moved qemu(), main.rs::mount() and chroot() to the tools module.
* Moved constants in main.rs to constants.rs (including base packages
  array)
* Renamed Presets struct to PresetsCollection to avoid confusion with
  Preset struct
* Moved main() to the top of main.rs to highlight general logic path
* Added comments and docstrings to some functions
* Removed some uses of `use foo::*` to make the source of imported functions
  and structs clearer

TODO:
* Move remaining code in main.rs to modules (except main())
* Break up create() function in to separate steps
* Log every command run (with arguments) to debug! when verbose flag is used
* Add docstrings for remaining functions and document constants (e.g.
  why noatime is used)
* Remove remaining uses of `use foo::*`
* Consider renaming/moving tools module to address tool:: vs. Tool::
  confusion

src/args.rs

@@ -2,6 +2,8 @@ use byte_unit::Byte;
 use std::path::PathBuf;
 use structopt::StructOpt;
 
+/// Parse size argument as bytes
+/// e.g. 10GB, 10GiB, etc.
 fn parse_bytes(src: &str) -> Result<Byte, &'static str> {
     Byte::from_str(src).map_err(|_| "Invalid image size")
 }