Added expect for improved readability

[xtay2]

Both the tokenizer and the parser frequently check if the next peeked token exists and equals some expected value.

I added two utility functions that should improve the readability.

[edy131109]

Great code but in tokenization.hpp please don't replace the peek().value() == '' with the expect function because we already know that the peek() has a value from the upper statement and that just makes the code slower.