Allow strings to close at the final character

The lexer was reporting strings whose close quote was the final
character in the code as unclosed. This now only happens when the lexer
index reaches the length of the text.

An additional escape when printing strings has also been added for tabs.
Add single-line comments

The lexer now processes and discards any line as soon as it encounters
'//'. The resulting token is treated as a newline.
Handle ignored parameters on request

Requesting a method or a block with ignored parameters halted the
interpreter, but the tests never requested either. The tests have been
updated to request their definitions.
Allow dot sequences to be operators

All dots were lexed as punctuation, which disallowed the range operator.
The lexer will now identify two or more dots in a row as a symbol
instead.