Make API simpler and smaller

[tertsdiepraam]

• We always want to use try_parse so remove parse and rename try_parse to parse. This makes the tests more verbose with unwrap but this is the behaviour we are gonna need in uutils, by making this implement UError and then we use ?.
• I've removed ArgumentIter from the public API, because it was only cluttering the docs and with some small changes did not need to be public.
• help now returns a string instead of writing to stdout.
• Make Value derive not fail without #[value] attributes

Closes #77
Closes #74