Allow initializing lexers with a character iterator

[osa1]

Currently in main branch and the versions released on crates.io, generated lexers need to be initialized with a &str argument for the input.

It's also useful to initialize the lexers with a Iterator<Item = char> + Clone argument, as I do in from_iter branch (used in my unannounced text editor project). The problem is without &'input str we can't return a &'input str for the current match.

Instead what we can return in match_ is the byte and character bounds of the current match. The user can then extract a string from the input if they have it as a &str.

Another alternative could be to maintain a String buffer in generated lexers, and return a slice to it (possibly with character and byte bounds of the match). The function will look like fn match_(&self) -> (&str, ... char and byte bounds ...). Users can then clone the string if they need to store it, or ignore it.

I think the first alternative (don't return a slice or string, just byte and char bounds) may be good enough. The the user has the input as a string, they can use the byte indices returned by match_ to get the slice of the match. So it's possible to get the current implementation using a lexer that takes Iterator<Item = char> + Clone as the input stream.

[osa1]

I just realized that we already have the method match_loc for getting the start and end locations (byte and char indices) of a match.

I think we could avoid breaking backwards compatibility by adding a codegen option to the macro for optionally (opt-in) generating the lexer with a Iterator<Item = char> + Clone argument. Changes needed:

• Add a type parameter to lexgen_util::Lexer for the iterator: I: Iterator<Item = char> + Clone
• Clone the iterator and store the cloned iterator in last_match field
• When backtracking, use the popped iterator from last_match instead of the input field
• Add this constructor to lexgen_util::Lexer:

  impl<T, S, E, W, I: Iterator<Item = char> + Clone> Lexer<'static, T, S, E, W, I> {
    pub fn new_from_iter(iter: I) -> Self {
        ...
    }
  }

• Existing constructors should be modified to use std::str::Chars as the I argument
• In the generated code, generate constructors based on the user-specified initializer type, and do not generate a match_ method if the iterator initializer is selected.