Clean up lexer

[kevlarr]

Lexer currently makes too many decisions about valid syntax when it should just be collecting valid tokens. It shouldn't care that some@ thing isn't a valid fragment.

Additionally, if it allowed some@ thing then it would emit the same tokens as some @thing since it scrubs whitespace, so there should most likely be a Token::Whitespace to represent any contiguous span of spaces and/or tabs.

It probably shouldn't also add a Newline unless there is one.