Test lexer / remove unused functionality

[Ploppz]

Generally need tests for the lexer module.

next_word in parser/lexer/mod.rs:

/// Used by next, peek and back - returns substring and new position
/// If forward, places pointer at the next non-whitespace character.
/// If backward, places pointer at the start of the current word.

Needs a test to validate this.

And I don't think that the "previous lexeme"/back functionality is used anyway. Anyway - where back() is used, we can probably use peek() instead.