Add lexer example

[felipe-bonato]

Lexer is a classical example of usecase for coroutines.

This is a very simple and basic lexer that can lex single digit integers, "+" and "-".
Like these:

1+1
9-5+3

The example would be better if we could return values when we yield (kind of like a generator).
But it is what it is.

[rexim]

@felipe-bonato oh, that's actually super cool! Thank you! I'll think about returning values from yield in the meantime.