Replace spec indices search with regex

[kossnikita]

This allows you to search for indices regardless of the order of [], * and ?
Causes panic if the expression does not match the regex. For example, if it contains no or more than one token. This can be avoided if the function returns an option.

[burrbull]

Could you add several examples of supported expressions inside spec_ind docs?

[burrbull]

1. Lets replace match panic with PatchResult.
2. It's better to compile regexpr once: see

   svdtools/src/html/html_cli.rs

   Line 30 in 9ab01ef

   static RE: Lazy<Regex> = Lazy::new(|| Regex::new(r"\s*\n\s*").unwrap());

3. Fix example. See CI error

[kossnikita]

About tests, I'm not very good at this. I can't use private functions in doctest? If not can I just use compile_fail for this?

[burrbull]

You can just ignore it: https://doc.rust-lang.org/rustdoc/write-documentation/documentation-tests.html#attributes