More readable formatting and identifier names.

[msmorgan]

This macro provides a a great example for developers
learning macros to study. It's easy to discover, since
libstd and many other crates depend on it, and the
macro itself illustrates a number of useful concepts.
This commit reformats some code to make the brackets
easier to see, and renames a few identifiers for clarity.

Additionally, the + and ? repetition quantifiers have
been used where appropriate instead of *, and an extra
( () () ) item has been removed, which had no effect.

[alexcrichton]

Looks great to me, thanks!

[chris-morgan]

I was just reviewing this crate and thought it worthwhile to note two things about this:

• Changing that * to $(…)else+ is a breaking change, even if it is of unintended behaviour (see else can be used without if #23). I recommend reverting that one character change, because I can imagine realistic code that it will break, or calling the next release 2.0.0.
• Changing that other * to ? changes no functionality but increases the minimum supported Rust version from probably 1.31.0 to 1.32.0. (1.31.0 only because it’s marked edition 2018; otherwise I think it’d run on 1.0.0; I can’t remember what Cargo did back at 1.0.0 with unknown keys like edition, whether it errored or allowed them.)