Prefer single quotes for terminals

[CrockAgile]

An even more silly PR, but maybe a style question. Most of BNF's examples and tests use double quotes for the string literal of terminals. This leads to a lot of escaping, and takes more space than a single quote (\" vs ').

This PR changes examples and tests to use single quotes. I don't think this matters much, but it does look better to my arbitrary eyes/brain.

The one controversial part to his that I see is that the existing Display implementations default to double quotes. So users who call Grammar.to_string() or similar will see the terminals in double quotes.

An alternative to this PR would be to update examples / tests to use r# string literals. This would allow for double quotes, without the need to escape:

r#"<zip-part> ::= <town-name> "," <state-code> <ZIP-code> <EOL>"#

[coveralls]

Coverage: 91.063%. Remained the same when pulling d6b5254 on prefer-single-quotes into 50cfb26 on main.

[shnewto]

oh mannnn this is so great. aside from it being great cleanup feeling, and even tho I think it's good we support both, I'm glad abt the idea that anyone reading the docs will be set up w ' as a place to start instead of "\"