Function sipmsg::common::bnfcore::is_alpha

pub fn is_alpha(c: u8) -> bool

Basic common rules from https://tools.ietf.org/html/rfc2234#section-6.1 and https://tools.ietf.org/html/rfc3261#section-25 that can be described by a limited number of characters ALPHA = %x41-5A / %x61-7A ; A-Z / a-z