Allow negative positions in String.substr method

[dalexeev]

The left and right methods support negative values.

Current behavior:

String substr(from: int, len: int = -1) const

Returns part of the string from the position from with length len. Argument len is optional and using -1
will return remaining characters from given position.

I think it makes sense to allow negative from values, but I'm not sure about len.

[h0lley]

negative from to go from the end of a string is what I'd expect from a substr function.

would negative len mean x characters to the left of the from position?
in that case the from should probably just be offset to the left by that number, and then len remain positive.
also it would conflict with the "return remaining characters" feature.

both for negative from and len, if turned into a proposal, you'd have to reply to "can it be worked around with a few lines of script?" with "yes" though. but I don't think that should immediately exclude it, especially when virtually all substr implementations of comparable languages support negative from.