bug(syntax): Panic when `Parser` is used twice

[langston-barrett]

What version of regex are you using?

0.8.11

Describe the bug at a high level.

Parser panics if it is used twice

What are the steps to reproduce the behavior?

fn main() {
    let mut p = regex_syntax::ast::parse::Parser::new();
    p.parse("a").unwrap(); // ok
    p.parse("a");          // panics
}

What is the actual behavior?

thread 'main' (13) panicked at /playground/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/regex-syntax-0.8.11/src/ast/parse.rs:983:9:
assertion `left == right` failed: parser can only be used once
  left: 1
 right: 0

What is the expected behavior?

Ok(...), because there is no documented panic: https://docs.rs/regex-syntax/latest/regex_syntax/struct.Parser.html#method.parse