Allow partial parsing from string about bnf HOT 3 CLOSED

@CrockAgile I agree that this would be a really good add. I also think that we resolve your concern by requiring the same input format all they way down so we can use all the existing parsers, like grammar to string requires nonterminals to be delimited by < and > and terminals to be delimited by " and ". Can we just expect that for the others? For instance:
revise let expression = Expression::from_str("nonterminal terminal").unwrap(); to
let expression = Expression::from_str("<nonterminal> \"terminal\"").unwrap();

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@Snewt That makes so much sense. Just realized I was thinking about terminals totally wrong. Wooo that makes this very doable. I'll count my question as resolved then and add this to 0.2.0 milestone

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Closed by PR #24

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