Grammar input type requires a ".len()" method

[adrianwn]

The generated parser code calls the len() method on the grammar input, but the Parse trait doesn't contain such a function. This limits the types that can be used as input for the parser. I can think of two ways to fix this:

1. Add a len() function to the Parse trait.
2. I believe there are only two places where len() is called on the input, and both are conditions in the form of __pos == __input.len(). So instead of adding a len() trait function, I suggest fn is_eof(&self, p: usize) -> bool, which can easily be implemented for any type that already has a len() method.

What do you think? I'd be happy to provide a pull request, if desired.

[kevinmehall]

Yeah, all the methods called on input should go through the traits -- thanks for pointing that out.

Option 2 sounds good to me.