This prefixes all non-builtin rules with r# to allow for use of rust keywords as pest rules.

This refactors code to use format_ident! rather than Ident::new (with format! internally in some cases), as it does the same thing internally. Span is defined to fallback to Span::call_site() in case of being given a non-ident, so there shouldn't be any issues.

Updated generate_complete test case and removed the rust keyword restriction from the validator.

* Updated tests that use identifiers to escape them with `r#`

* Changed the previously added `generate_complete` `r#if` test to ensure using an identifier as an expression is properly emitted

* Removed some redundant Ident::new() inside of `format_ident!` calls

`generate_rule` was checking the name after formatting for whether it was `WHITESPACE` or `COMMENT`.

Re-imported proc_macro2::Span for tests since they still use it.