Add length primitive
Add first, rest primitives and nil symbol
No newline at the end of prompt
Add bool and nil printers

Display message when there is no printer for returned type.
Lexer handles readers
Load builtins at start up

Add empty? function.
Extract methods
Add zero? builtin
Add let form
Add lexical binding test
doc: concat lists, str strings
doc: & should be a symbol
Implement quoting expressions

For now only ' and quote function work. Quasiquote is not
yet implemented.
length -> count
Concat primitive
Add cons and conj
doc: I won't implement ` ~ ~@ soon
doc: mention quote function
doc: mention nil symbol
doc: str and print can operate on arbitrary types