v2.3.1

A fresh bugfix for ABS!

#402 fixes a nasty bug with silent returns (return statements without a value, which default to NULL).

The PR fixes them and adds some more sophisticated tests. A return without a value, when interpreted before this PR, would terminate the script without executing the rest of the code.

This snippet:

if false {
	return
}
return 3

would return NULL instead of 3.