0.4.0

• Added: initializer overloads of Prefix and other parsers that aid in type inference. E.g., the following now type-checks:

  Prefix { $0 == " " }
  // Inferred to be "Prefix<Substring>"
  
  Prefix { $0 == .init(ascii: " ") }
  // Inferred to be "Prefix<Substring.UTF8View>"

• Added: CharacterSet now conforms to Parser.
• Changed: Many will now trigger a warning breakpoint when its upstream parser succeeds but fails to consume any data, which can cause an infinite loop.
• Fixed: Double.parser() exponent parsing (thanks @bradhowes) and general Double parsing edge cases.
• Infrastructure: documentation fixes (thanks @igor1309).
• Infrastructure: improved "arithmetic" demo parser to use more ergonomic/performant InfixOperator parser.
• Infrastructure: benchmarks now better reflect typical parsing (substring UTF8 views vs. unsafe buffer pointers).