Replace accept_if function with next_if, eat_char and eat_not_char methods

[bjorn3]

This makes the call sites slightly clearer and takes advantage of Peekable::next_if instead of manually reimplementing it.

[bjorn3]

I noticed that the accept_if function functioned the same as the Peekable::next_if method while reviewing #897

[squell]

Since the trait CharStream was removed, that is the case, yes. I do slightly prefer the name accept_if over next_if, but the introduction of eat_char is a terrific improvement for most of the cases accept_if was used (and next_if is a standard function that we can assume familiarity with). So thumbs up for next_if and eat_char.

However, I do not like the eat_not_char function name. I also notice it's only ever used as while eat_not_char('\n'). Since it's unlikely we'll ever use eat_not_char for anything except error recovery/ignoring input, let's just introduce a function ignore_until_eol, skip_to_newline, ignore_rest_of_line, or something similar).