Add 'static' analysis passes to `zero-copy`

[zesterer]

It would be useful to have a way to introspect the parser to catch common problems and suggest improvements. See this branch for an example of how such passes could be implemented.

Useful passes:

• Discover instances of repeated/separated_by that operate on parsers that can parse no inputs (i.e: catching this case)
• Discover inefficient patterns (prefix.then(a).or(prefix.then(b)) instead of prefix.then(a.or(b)), for example)
• Discover patterns that could be simplified (a parser that emits () having .ignored() applied to it, for example)
• Discover redundant parser logic (i.e: empty().ignore_then(x))
• Identify very large parser types, suggest use of .boxed() (recurse through parser and sub-types, count nodes)
• Generate BNF-like grammars (or other syntax schemas) automatically from combinators

Additionally, it would be useful to have some sort of profiling tool build into the crate, such as a way for individual parsers to record how many times they're being invoked to discover bottlenecks in the parser logic.

[CraftSpider]

Personally, I think there may be some worth to having some kind of 'visitor' or something - maybe an API like the following? We can make it internal for now and expose it only through other methods if we want.

/// We provide both 'enter' and 'leave' so we know when we're moving up or down the tree. There are alternative ways to handle
/// this also.
pub(crate) trait ParserVisitor {
    fn enter_parser(&mut self, info: ParserInfo);
    fn leave_parser(&mut self, info: ParserInfo);
}

pub(crate) trait Visitable {
    /// This should be implemented as roughly `visitor.enter_parser(...); self.parser.visit(...); visitor.leave_parser(...);`
    fn visit<V: ParserVisitor>(&self, visitor: V);
}

pub(crate) struct ParserInfo {
    name: &str,
    size_hint: (usize, Option<usize>),
    // Whatever other fields we find useful
    ...
}

[zesterer]

I can certainly see this being a useful way to approach this feature.

[CraftSpider]

Trying out an API like I described - seeing how well your implementation converts