Fix bug in StdStream.print

[leonardoce]

When printing via StdStream.print strings containing the null
terminator, we were just printing the string until the null terminator
and replacing the unprinted characters by padding the printed string
with space characters.

This behavior made String.size() inconsistent with what fprintf was
really printing.

This behavior has been introduced in #1768, which ensure we respected
the buffer size.

Closes #4171

[leonardoce]

Hi guys, this is my first PR here and I'm not totally sure this is the behavior we want.
Any advise is deeply appreciated.
Thank you!

[ponylang-main]

Hi @leonardoce,

The changelog - fixed label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

Release notes are added by creating a uniquely named file in the .release-notes directory. We suggest you call the file 4180.md to match the number of this pull request.

The basic format of the release notes (using markdown) should be:

## Title

End user description of changes, why it's important,
problems it solves etc.

If a breaking change, make sure to include 1 or more
examples what code would look like prior to this change
and how to update it to work after this change.

Thanks.

[leonardoce]

Added the release notes snippet

[SeanTAllen]

I'm not sure what this is supposed to mean. What is "effectively printing"? Does "effectively" here mean "more or less"? If yes, that would be a disturbing release note and I think it should be rewritten.

I would rewrite this, but I'm not sure what the intent is...

[SeanTAllen]

We don't have a way to test this right now in an automated fashion so I will build this branch and test by hand:

actor Main
  new create(env: Env) =>
    env.out.print("Hello,\x00world!")

[SeanTAllen]

Tested. This is looking good.

[SeanTAllen]

I'll merge this later.

[SeanTAllen]

Thanks @leonardoce.