Fix stdout.print/stderr.print panicking

stdout.print and stderr.print perform two writes: one for the message, and one for a newline. The newline write was not wrapped in a "try" expression, causing any exceptions thrown to go unnoticed.

Sending messages to optional types inside a "try" was also broken. When doing so, the VM would jump to the incorrect instruction and effectively ignore the "else" clause; bubbling up the error instead. This is fixed by having the compiler generate different code: instead of jumping to the next block (whatever that may be), we jump to a specific block.

As part of this the signature of stdout.print/stderr.print is also changed. Instead of the argument being typed as ?ToString, it's now a ToString with an empty String as the default value.

This fixes #199 (closed)