Provide a way to call Lexer methods from SpannedIter

[simvux]

Do you think this is appropriate or would it be better to just manually re-create some of the useful methods/impls such as Clone source() slice()?

[marcospb19]

would it be better to just manually re-create some of the useful methods

I prefer this approach because someone might add some method to Lexer, and forget to add it to SpannedIter.

I'd love this to be merged, ping @maciejhirsz.

[simvux]

I've accidentally done some other stuff on the same branch since I forgot about this PR. But if the project wakes up again then I'll just make a new PR.

[maciejhirsz]

Hey! Instead of as_lexer and as_lexer_mut could we implement std::ops::Deref and std::ops::DerefMut instead? That should make it possible to access all the Lexer methods (sans the ones that take Lexer by value) and fields more ergonomically.