Parser cleanup (implement shortcut methods, use new parse oneOf)

[Davidde94]

Resolves #476

Also works to improve the parser errors as mentioned in #483

We now have shortcut functions to the parser library, meaning we don't need ParserLibrary to appear every line.

Also any instances of calls to oneOf where there were 2 or 3 parser functions have been replaced with the new multiple-argument method.

Result:

• NIOIMAPPerformanceTester is about 20% faster
• If we added a new 4-option oneOf parser it hits about 30%, but we need to draw the line somewhere.

[Davidde94]

@swift-server-bot test perf please